There are some countries where linking (in a webpage, web forum, news forum or such) to illegal material is illegal. In other words, just putting a link to illegal material is illegal in itself. As far as I know, this is so at least in the US and Australia, probably in some other countries too.
The big problem with this is that these laws are really fuzzy and they really don't make any sense.
If I post somewhere a link which points directly to an AVI file which contains a copyright-violating video, that's illegal. Seemingly this is so even if I post a link to a webpage which contains a link to such an AVI.
However, what happens if I just post a link to a webpage which only contains links to "illegal" sites? What happens if someone links to this post of mine? Is he doing something illegal too? The number of indirect links can go on forever. How many indirection steps must exist before it becomes legal? One? Two? Ten?
What happens if I don't give a link to the illegal material, but instead I give verbal instructions on how to find it, for example like "write this search string in google and go to the result which says this and this"?
What happens if person A puts in his homepage a link to the homepage of person B, which contains only legal material, and then much later person B puts illegal material in his homepage, and person A never notices? Is person A a criminal?
And more generically: What happens if illegal material appears in the target site after the link to that site has been posted somewhere? Does the link become illegal and the poster a criminal? How can the poster prove that the site did not contain illegal material at the time he posted the link?
How can someone even know that something is illegal? If a website has, for example, some video in it, and there's absolutely no indication nor hint that the website actually doesn't have rights to distribute that video, how could the average person know that? He could very well post a link to the site without knowing that it's illegal.
The general answer to all these questions is: "It hasn't been tested on court yet." And that's exactly why this kind of law sucks big time: It's fuzzy, unclear and doesn't state specifically and unambiguously what is and isn't legal. How can anyone obey a law which is so unclear and ambiguous? How can it be fair to convict someone on the basis of such a fuzzy law?