Never heard of it either. Only way I can see it happening is if someone was really drunk.

As an American, I've never done it or known anyone who had admitted to it.

I wonder if wearing your pants on your hips or waist makes a difference. Maybe it's more likely to happen to smaller penises. Or maybe it's a puritan holdover, like, "gotta put my penis away as fast as possible."

I also noticed in american media that people only open the zipper to take a piss, but not the button - maybe that's the difference?

It's apparently common enough that the military has a specific tool in basic training med-whatevers for cutting off the zipper.

I honestly never even conceived of the notion until I read that fact as a young adult.