No matter how much damage you get, you will always be able to function at full capacity. It doesn't matter if you have been sliced by a chainsaw, shot by a shotgun or stabbed with a katana, you will still have full capacity like when completely healthy. Even if you have 1% of health, you will still be as good as new. But of course if your health meter drops to zero, you just suddenly die.
Corollary: Unfortunately your enemies possess this exact same property.
No matter how much damage you get, it will be immediately restored with a health pack. Sometimes even food can do that. No recovery time is required.
Corollary: Fortunately your enemies do not possess this capability. Any damage they get will usually be permanent (even if they happen to walk over that health pack).
It doesn't matter which part of your body the enemy projectile hits: It will always cause the same amount of damage.
Jumping is an immediate process which does not need preparation (ie. crouching). You are simply magically boosted upwards by an unknown force.
This force will usually boost you much higher than any normal human being would be able to jump, even if you are supposed to be one.
You will also usually be able to fall from much bigger heights than a normal human being would. At most you will get some damage (which, due to the law of limited invulnerability, will not affect your functions in any way).
Usually you can fall in water from any height and get no damage whatsoever, no matter how high the fall is or how shallow the water.
No matter how far you are thrown by an explosion or how high you fall from or how hard you are hit by enemy fire, you will always be able to stand perfectly vertical, without ever falling to the ground or even having to regain your balance.
Corollary: You will never drop your weapon either.
Usually you are able to swim and to dive regardless of how much armor, equipment and weapons you are carrying. Often you are even able to jump while swimming. Weapons will work underwater equally well as above it.
Usually you can possess an unlimited amount of items and weapons, no matter how big and heavy they are. All these items are stored in hammerspace and thus don't take any real-world space nor burden you with their weight.
The portal to hammerspace is usually the lower edge of the screen: This is used mainly to switch weapons.
Most items (such as collectible health packs) can be used directly from hammerspace, without taking them out first.
Even though you can have any amount of weapons, no matter how big and heavy they are, for some reason you can only have a finite (and usually quite small) amount of ammo for those weapons. Trying to collect more ammo than this limit won't work.
Sometimes other items are also subject to such limitations.
For unexplained reasons there will always be useful items laying around any place you are, even if these items are in no way related to that place in question (for example plasma rifle ammunition in a public library). Even if the items are somehow related to the place, they will nevertheless be laying around at random places, usually on the floor, instead of being stored meticulously in the places where such items usually are stored.
No matter how exotic weapons you have, even if they are newly-created prototypes, there will always be ammunition for them laying around, even at places which have nothing to do with the weapons in question.
If at some point there is an area in your path with an unusually large amount of ammunition, health packs and other similar useful items, and especially if the area is completely devoid of enemies so that you can collect all these items without being bothered, expect a big boss fight.
(There never is a storywise reason for such an ammo cluster to be there. It just is, without explanation.)
Exception: Sometimes such clusters appear after a big boss fight, as a reward, and thus are not subject to this law.
Variation: Sometimes instead of an ammo cluster there's an infinite ammo suply. Expect a big boss fight, and having to refill from that supply many times during it.
Even though you can run out of ammunition, your enemies can't. They have a magical infinite ammunition supply.
Corollary: Sometimes one of your own weapons has infinite ammo. Usually this weapon is the least powerful and least useful, though.
Corollary 2: If a weapon is fixed to the ground and cannot be taken (but can be used), it has an infinite source of ammo. This is because it's an enemy weapon and thus subject to the above law.
Reloading half-full magazines is very easy and fast. This is so even if the magazines contain something more intangible than physical bullets (such as plasma charges).
If bullets are stored in magazines (instead of individually), filling a half-full magazine in the weapon with the ammunition from another half-full magazine (in hammerspace) is equally non-problematic. The on-screen animation will, however, always show you changing the entire magazine.
Collecting an item on the ground is very easy: Just touch it with any part of your body. The item will be magically teleported to your hammerspace inventory (except if the limit for that specific item in hammerspace is full, in which case the item will not be teleported).
If the item is something with an immediate effect (such as a health pack), just touching it triggers this effect on you.
If an item has a case which contains the useful stuff inside, after using the stuff the case will simply vanish.
If you collect a weapon you already have, you will simply collect its ammunition. The weapon itself will vanish.
Corollary: There are never unloaded weapons anywhere. Never.
No matter what kind of weapon or item you get, even if it's a completely innovative new prototype weapon, and no matter how complex the item or weapon is, you will automatically know how to use it.
Often you have an armor which has the same kind of status as your health. Damage weakens the armor. However, like your health, collecting an armor item will immediately repair the damage.
Your enemies will usually blissfully ignore any items laying around, even if those items would be beneficial to them.
Corollary: The reason for this might be that they don't possess a hammerspace nor the item teleportation ability.
Perhaps for the previous reason enemies usually have just one weapon, period (even though you can have a dozen of them).
There are always crates and barrels everywhere. Always.
At least some of the barrels laying around will explode if you shoot them enough times. No logical explanation exists why there are so many exploding barrels laying around in city streets, hi-tec building complexes, etc.
No matter how big the crates are, there will never be pallets below them nor other visible clues about how they are lifted, making it a mystery how the crates were transported there in the first place.
Likewise, it often happens that the crates are inside rooms with doors (and possible windows) which are narrower than the crates themselves, making their presence in the room even more mysterious. (Were they built inside the room?)
Some crates may have items inside them which can be collected by breaking the crate. For some mysterious reason the crate is usually enormously oversized compared to the items inside it.
Corollary: Sometimes crates are just empty. Why there are so many empty crates laying around is a mystery.
If you want what is inside a crate, you'll have to destroy the crate. You can't simply open it. This is so even if you have a crowbar, which would be the perfect tool designed for that exact purpose.
Most of the scenery will be completely indestructible. No matter how powerful weapons you have, even if these weapons can be used to destroy armored vehicles and enormously oversized alien monsters, trying to destroy a wall, or even a window or a wooden door is usually impossible.
Even a simple locked wooden door is enough to completely stop you from getting to the other side without a key. It doesn't matter if you try to destroy the door with a bazooka or a plasma cannon, it won't budge. Never mind that this exact same weapon will be used to destroy armored vehicles and megamonsters.
Corollary: Trying to bypass the door by going through a window besides it will also by futile. The window is also protected by this indestructibility.
No matter how fortified a place is or how much security there is to stop invaders from passing through, there will always be a way. Indestructible doors will always have a key which can be collected from the side where you are in (such keys will never be on the other side which is completely inaccessible without it). Also, no matter how secure an installation is, there will always be an easy way of opening obstables by pressing a button or touching a computer screen. If nothing else, there will at least be a helpful NPC which will open the door for you.
Obstacles (such as doors) which can be bypassed will usually have visual clues which makes them obvious and clearly distinguishes them from similar obstacles which are just part of the scenery and play no useful role.
Usually there are only two choices where to go next: Forwards or backwards, and the latter usually doesn't end up anywhere. Even if the path seems to split, the "wrong" path will be relatively short and contain at most some items to collect.
In scenarios where there would normally be multitude of choices (such as being in the middle of a big city), all the "wrong" choices will for some reason be blocked, for example, by rubble, wreckage, locked or broken doors, or even just parked cars (which cannot be jumped over).
Your enemies will usually be strategically located in the map, and will patiently wait there for your arrival, no matter how long it takes.
Sometimes when you are forced to backtrack a level because of story scripting reasons, new enemies will appear at places which you had already cleared. Often it's not at all clear how they got there.
Corollary: If you skipped killing some enemies the first time, when you backtrack (because of forced scripting) those enemies will have usually vanished for some reason.
Most enemies will run right towards you, no matter how much you are firing at them (although there are some exceptions to this).
They will usually also fire right towards your current location, even if the projectiles are slow and even if you are clearly moving sideways.
Enemies firing slow explosive projectiles (such as rocket-propelled grenades) will always fire them right towards your face. It never occurs to them to fire at the ground where you are standing (making the projectile explode, causing damage even if you get to move some distance away). Thus these projectiles are laughably easy to dodge (but be careful to not to be standing near a wall).
Even if an enemy is smart enough to hide behind a crate, they are still stupid enough to not to run away if you fire/throw an explosive towards them.
Most weapons lack recoil completely. You can, for example, shoot several shots per second with a fully-automatic rifle, all of them exactly to the same tiny spot.
Corollary: You are usually also able to hold your aim flawlessly on the precise tiny point indefinitely, even if you are using a telescopic sight and the target is very far away. (Exceptions exist to this, but they are rare.)
Weapons never jam nor malfunction in any way no matter how much they are used. Rifles using telescopic sights will always be perfectly aligned.
Bullets always travel straight, without being affected by gravity. They also usually travel infinitely fast, without any delay between the shot and the hit, yet this doesn't cause any recoil whatsoever.
Corollary: The only possible explanation for this is that bullets have no mass and are completely impervious to air resistance. On the other hand, how massless bullets cause damage is unexplained.
Even though you sometimes can see your own hands, if you look down you can't see your feet or body. If the game is so advanced that everything casts shadows, you will be the only thing which will not.
Partial invisibility does not apply to reflections in mirrors (but does to ones on surfaces with blurred reflections).
Any simple tasks requiring the use of hands will usually be impossible. These include things like climbing chest-high walls or catching the edge of a pit to avoid a fall.
Corollary: Climbing ladders will usually be done without hands, while still holding your weapon (even if holding the weapon requires both hands).
All enemies of the same type will be perfect clones of each other. They will look the same, they will behave in the same way and they will have identical voices (and if they speak, usually they all say the same sentences from a small selection).
The more enemies attack you at once, the weaker each single one of them is. However, an enemy which attacks you completely alone will usually be really hard to defeat.
Exception: Sometimes weak lone enemies are randomly scattered along your route just to keep you busy.
If the same strong lone enemy is encountered again much later in the game, it will be then much easier to defeat than the first time.
This is, however, often compensated by having more than one of them attack at the same time, returning the phenomenon to the regular stormtrooper effect.
The stormtrooper effect works in the other direction as well:
Whatever the situation may be, be it an alien invasion (or just a regular army invasion), a post-apocalyptic dystopia ruled by an evil dictator, a monster infestation or whatever, entire full-fledged human armies will be completely helpless against these foes, but a lone person (you) will be able to single-handedly destroy the entirety of the invading forces, penetrate their fortresses and destroy their leaders, no matter how powerful they are.
In short, a "one-man army" will be more powerful and effective (sometimes even without special training) than a full-fledged army of highly-trained soldiers.
The surrounding universe outside of the playing area is located at a different time level than the area itself. This means that no matter how long you play in that area, if the sky is visible, time of day will be static. Especially if the Sun is visible, it will not move no matter how many hours you play.
In a few cases the phenomenon is the opposite: Time outside the area will travel must faster than inside. If there is day and night, sunrises and sunsets, a whole day will usually take something like 30 minutes.
If the game has a graphical inventory, accessing it effectively stops time for the entire universe except for the main protagonist, who can browse and rearrange his inventory for as long as he likes. Even under heavy fire and grave danger, the protagonist can suddenly spend ten minutes in his inventory without problems. The battle will continue after he's done.
Sometimes accessing computers and other similar devices has the same time-stopping effect.
No matter how many hours you run, jump, swim and perform other physical activities without rest, you will never get tired, you will never have to eat nor drink nor go to the bathroom and you will never have to sleep.
If you encounter an elevator, it will usually have only one button, or at the very most two (up and down). This even if the elevator is inside a skyscraper.
In the extremely unlikely case that the elevator actually has a realistic number of buttons, only one of them will work nevertheless. The rest will be broken or somehow disabled (or not selectable).