Among few not worth mentioning mistakes, I have one that is really funny to me now: few years ago when I became regular visitor on perlmonks.org (but not registered yet) I thought that an Anonymous Monk was the real user, and I thought: 'What a cool and imaginative name'.
Later I noticed some obvious contradictions in several of his/her posts, so I did some research (read help docs, etc), and when the truth came to me I just couldn't believe it :-)