The constant fridge-raiding during parties. It's so bad that I have to lock a door and put birthday cakes in a room no one can access so some stupid sim doesn't go and take a slice from the birthday cake before the candles are lit, or the "eat cake" thing pops up. It also annoys me that I can have plates of excellent food out on the table or counter, but no, they go into the fridge to eat the leftovers there first.
Non-household sims using the computer, or any sim using a computer during a social event autonomously. If the computers aren't in a room that can be locked, I end up moving desk chairs to the inventory to keep them off of them. I've done this with the easel, too, when visiting sims think it's fine to start creating a painting in someone else's house.
Sims that go to bed in the middle of their own parties. The second the bar turns yellow, they go off to bed with a house full of guests? No. Get some coffee and suck it up, buttercup. Don't be rude. (I'm so glad door locks can fix this as well, but it's sad that I have to lock the bedroom doors to prevent this!)