What error bugs you most?

For me, a misplaced apostrophe makes my face red. I go ballistic when I see sock's for sale or "visitor's hours," as if there will be only one visitor.
Apostrophes are little things but they do make a difference. We mustn’t be so careless; we must make an effort. Apostrophes should be our friends and we should take care of them. We don’t want to leave them hanging where they don’t belong or leave them out when we should include them.