I admit I was laughing too....
Correction: I was laughing so hard I thought my sides were going to burst.
But I started to think how funny it is for the one being played the prank? I know for one, if I'm doing #2 I would be mortified if someone in the next room could hear me... let alone see me.
I like a good laugh and I've done pranks on people that well, are best left to another thread (and to an entirely different venue), but really thinking about it, I don't quite see the humor in humiliating someone when they're on the loo?