Why eyewitness testimony is almost worthless

One very common feature of people who believe in UFOs, supernatural entities (such as ghosts), psychic powers and so on, is their complete overreliance on eyewitness testimony. Most of these people put significantly more weight on it than they should, and there's probably not a single book or pseudodocumentary on the subject that doesn't extensively present eyewitness testimony as evidence of the phenomenon.

What most people (even including those who don't believe in such things) don't realize is that eyewitness testimony is not only unreliable, it's actually almost completely worthless. At most it can be used as a startup for investigation of the described phenomenon, but not as any kind of evidence.

Also, it doesn't matter how reliable the eyewitness is. It's very common to confuse these two things. In other words, the reliability of the person (in other words, how honest he is, how educated, how experienced, and so on) is equalled to how reliable his eyewitness testimony is. While experts on the actual subjects being dealt with might be more credible than laymen, their eyewitness testimony is still almost worthless and cannot be taken as evidence of anything.

There are many reasons why eyewitness testimony is almost worthless when determining the veracity of a claim or phenomenon.

People very easily misinterpret what they see. Pareidolia often plays a big role in it. Othertimes it's just something that the person has never seen before and can't understand or figure it out. This has nothing to do with honesty, intelligence or experience in other subjects. Even the smartest and most honest people in the world can get confused and misinterpret what they see.
People very easily misremember things. This is much, much more common than people want to believe, especially from themselves. Almost no human has a photographic memory, and details get lost, confused or outright changed as the brain processes them over time. The person might be completely and absolutely sure to the highest degree of what he remembers, but it may nevertheless not be completely accurate. (Most people have personal experience of this very thing, yet they still don't want to believe it. When it sometimes happens, they prefer to think of it as a one-time thing, not a common trait in themselves.)
People are very unreliable at repeating what they have seen or heard. This is especially true when they are repeating what somebody else has told them. The reason for this is simple: People do not repeat every word they heard; instead, they describe the mental images they got when they were told the story. When someone hears (or reads) a story, he forms a mental image of what is being described, and that mental image is seldom accurate, and there will always be wild differences to the original event. When this mental image is later described, it may have surprisingly big changes to the original story.
People easily confuse second-hand testimony as personal experience, especially after a long time has passed. This is a combination of the previous two points: Somebody hears a story (possibly from a first-hand eyewitness), forms a mental image from it, and then years later misremembers and thinks that this mental image was actually what he personally witnessed. In other words, he forgets that he was told the story, and instead remembers only the mental image, and thinks that he witnessed the events himself. This is also surprisingly common, and something people don't want to believe.
People easily form false memories when "coached" by others. This "coaching" may be (and often is) completely inadvertent and unintentional. People around them talk about these things, he sees "documentaries" and reads books about the subject, and he inadvertedly forms mental images that might vagely resemble real experiences or not, and then later he, again, misremembers them as actual events. These events might have never actually happened at all, or it might have been something only very vaguely resembling them.
People uncritically repeat as fact what their most trusted friends or family members have told them as fact. People tend to be uncritical of people they trust. As noted, that other person might not be lying and might completely honestly believe that they are telling an accurate description of the event. However, that doesn't mean the story is completely true. People make mistakes all the time.
Sometimes people suffer outright hallucinations. This is, in fact, much more common than most people think, and it's not a sign of illness or mental defects, but can be a completely normal brain process, completely akin to dreaming. The only difference to dreaming is that it happens while being awake (although in most cases when very tired, falling asleep or just awoken). Most people recognize such hallucinations for what they are, but others overinterpret them (in most cases as ghosts, angels or demons, in some cases even as aliens).
Many people hate admitting being wrong, even to themselves. For this reason they will stick to their story even when it becomes clear that they are misremembering or misinterpreting. They will thus keep repeating the story to other people, ignoring the facts.
And then there are, of course, the outright hoaxes. People like to deliberately invent stories and even stage events just for the fun of it, to get attention, or in some cases for personal gain. However, deliberate deception is probably only a minority of the cases.

All of the above can actually be tested, and they have been tested countless times, always with the same reasult: Humans are unreliable recorders of actual events. Often the results surprise even the researchers by how wrong people can get things. Yet people still don't want to believe this. They still put way too much weight on eyewitness testimony.

It's very common for people to argue that they trust the person who told them about the extraordinary event. "He is my best friend, I know him, and he would never lie to me." However, in the majority of cases it's not a question of honesty. That friend may be completely honest, but that doesn't mean he is right. He may be honest, but he is human, and humans are completely unrealiable eyewitnesses.

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