It really annoys me when some guy doesn't lock the door of a public restroom, and I walk in and see him on the crapper. How hard is it to lock the damned door!?
I grew up in a house where my dad and older brother just walked in to the bathroom even when the light was on, problem was the lock on the bathroom door was weak, and any hard thrust make it release, so they were always walking in on my despite my best effort to lock it.
I eventually put a deadbolt on the door, and did the same for my bedroom since my brother did the exact same thing there. I kinda now can't sleep unless the room I'm in has some kind of lock.