The kitchen faucet. Last week TheBF patiently pleaded with me that this is his biggest pet peeve, I only have this one thing, please stop doing it. It annoys him when I leave the kitchen faucet on "spray" instead of "stream" because his water pressure is so high that if you flip the lever to turn on the water 100%, it splatters water all over the place. It doesn't bug me because I don't automatically flip the lever all the way up, but I recognize that doing that isn't ergonomic. It's second nature when you're doing something in th kitchen to just flip that lever up all the way, and then you get sprayed with water.
I've been trying to remember not to do it because of how sweetly he pleaded with me to stop doing it.
It also bothers him when I open a lot of tabs on my browser. I don't know why because it's not like he uses my computer, but it really bugs him.