Two things that drive me absolutely nuts:
1) Blog posts that post, but don't post. It is both there, and not there. A blog post caught in a horrible transporter malfunction, and no way to reverse the polarity.
2) People unable to chew gum with thier mouths closed. Especially those who close proximity is inescapable, like travel companions, or cube-mates at work. I'm sure your gum is delicious, but I don't want to share it.