Grammar, spelling and punctuation errors. For some reason, when I see those things, I automatically assume people are 87% dumber than they probably are. This is a personal flaw; Jesus and I are working on it.
Therefore I ask: what's your biggest pet-peeve? Do you have any? Do you have several? When they occur, do you have the irresistible urge to punch someone in the face? Slam your head against the wall? Scream violently for several minutes? Or are you much calmer and less prone to dramatic responses?