Mine is when my students (or anyone) tries to talk for me, assuming they know what I am going to say next. I always change it up on them and say something different if I can, just to prove they were not "reading my mind." My middle school students think they know everything. It gets really old.
My other pet peeve is gum chewing... loudly. I don't chew gum because I cannot stand having to watch or listen to others.