One thing that really pisses me off is when I get a $100 medical bill from some hospital that I visited almost 2 years ago!
I have no idea what the deal is with this, but I'm guessing my insurance didn't cover everything, and the idiots are finally getting around to telling me about it now.
Another annoyance is when my girlfriend randomly decides to go into one of her "moods". No idea what triggers this stuff, and fortunately it only happens once every several months, but when it does, look out! Silent treatment and glares for a while, and she expects me to somehow figure out what the problem is.
It'll usually turn out to be something so dumb and inconsequential, that I wind up getting mad that she wasted 2 perfectly fine days being a butt dart.
Pet peeve: When people take their left turns on the green arrow SO SLOWLY that no one behind them can make the light as well, so they have to sit there for another 3 minutes before they get another chance. Also, when pedestrians casually walk along at a snail's pace across a drive way that you're trying to enter, while traffic piles up behind you. The pedestrian(s) could easily just pause for a couple seconds to let you turn in and allow traffic to continue to flow on the street behind you, before they continue on their merry way. Inconsiderate jerks.