After any time you've been out and have had too much to drink ( coffe, booze, anything liquid basically) and your bladder is so full that you go to the bathroom and need to go again immediately afterward.
Girl ( outside stall ) "Hurry up, there's a good song on and I want to go hear it."
Other girl ( peeing, in stall ) "I'm done, I just have to wipe.
Oh damn! I've gotta re-pee!"