In this article, the author writes:
"I've been thinking what "perfect AI" could mean in games. I started thinking AI in terms of "how naturally it behaves compared to what humans would do". I started thinking that there would be one crucial element that AI would need to have: the ability to make mistakes.
Making mistakes is natural to humans, but I believe making accidental mistakes isn't typical to computer AI. When computers make mistakes in games, they are more often because of "lack of better move" rather than "humanly mistake". Computer opponents in games are programmed to do wise moves, not to make mistakes.
So how would perfect AI play? I believe it would be a really tough opponent, but sometimes it could make small (even stupid) mistakes. Perhaps in FPS game the computer could make a stupid jump and end up in a lake (instead of the other side of the broken bridge). Or perhaps the computer could accidentally build wrong type structures in his base, and would need to re-build some of them. Or perhaps the computer could forget to heal some unit."