I really tried hard to resist this one, but I can't.
The thing that irritates me more than anything else is inappropriate or incorrect use of apostrophes, and there is at least one in this thread already!
Also:
People who expect others to clean or clear up after them.
Over-sensitive bank anti-fraud systems that actually prevent you from spending your own money.
Spelling and grammar mistakes on signs.
I am sure there are lots more, I just can't think right now. I will have to come back later.
