It bugs me when I come home and really need the bathroom, and I'm just in time to see someone else go in before me. And they were there first, so there's nothing I can say. We have another bathroom in the basement, one with just a toilet and a sink, and I've even had it where someone is in that one too. Upstairs, it's "Sorry!", then downstairs, "Sorry!", and I'm there, "Well, darn..."
I guess having someone get in ahead of me in general annoys me, because it seems to happen all the time, especially in the kitchen.
Me: *waiting on the coffee pot, all alone in the kitchen, and buttering some toast*
Mother: *comes into the kitchen, and begins fixing her breakfast in front of the coffee pot, effectively blocking it*
Me: *trying to reach around her for the coffee, which is now ready* Excuse me.
Mother: I'll be out of your way in just a second.
Me: *waits, anticipating cold toast and eggs when all is done* -.-'