I'm not going get into some big graphic disscussion about what goes on bathrooms. I'll leave that to experts other than myself. But I would like to comment on public bathrooms.
Here's the thing: When you're in the bathroom and somebody knocks on the door, you don't have to answer because you're on the toilet. Obviously, that's why you have the door locked. Otherwise the door wouldn't be locked, and you rude person on the other side of the door could come in freely.
I used to fall for this trick, and I'd yell to the person someone is in here. Well, no shit. That's why you can't come in you impatient bastard.
So my advice is "Don't Answer the Knock." In case they're impatient and stupid, they'll figure out somebody was in there as soon as I come out.
Wait your turn. And resist the knocking. Sometimes it's not opportunity. It's just stupid.