When you use a public restroom (e.g. at work) and are about to have a loud and embarrassing session, but another person comes in, causing you to hold it until they leave. After they leave you are free to let loose without fear of being laughed at.
Jim: Dude, you were gone for a long time.
Mike: Yeah, I had a nasty burrito for lunch and then had to out-sit 2 guys that came in to piss. I didn’t think I would make it!