Sometimes the problem is absolutely absurd. It could only be caused by one of the many bugs caused from pirating. Such as this thread:
http://www.frictionalgames.com/forum/thread-18783.html
I knew it was pirated because issues like this just don't occur from faulty graphics card drivers or the sort. Such as the huge list of
"WARNING: Particle system blah blah..." doesn't happen to a normal version of Amnesia. If a problem just looks, "strange"... Chances are it's pirated. When they're rare occurrences, it's more likely (this thread above was probably the only reported one on this forum).
Just be cautious of liars as well. When they say things like "got it from a friend", either:
1) They're lying, trying to hide their source.
2) The serial key is broken since their friend used it. We can't help them from that point, giving that free serial keys just don't happen...
3) The friend may have broken it or they pirated it just as well.