When you go to buy a piece of furniture, pay for it, and tell them "I'll be here tomorrow to pick this up." and they don't have it ready for pickup when you get there.
When your co-worker tells you that you can't use certain (common, everyday, clean) words at work. I got shit for saying "My best guess is...". Bitch, please. I can say whatever I want as long as it isn't rude or offensive.
What pisses me off the most? My mother in law. Stereotypical? Maybe. But goddammit, she's a menace. Smokes in my car, comes in to stay at our house with no warning, leaves her shit at my house, doesn't pay any attention to her grandkids (not my kids, we don't have any), blows off family functions to go golfing, spends all her money trying to promote her awful books that should have never been published, and comes in and uses all our laundry soap because she's too cheap to do her own laundry at home (without replacing it).