When two or more males gather around a single toilet or urinal to see how many times each flow of urine overlaps until all participating males' bladders empty.
"Hey, Chad. Want to go in the bathroom and Cross the Streams? Bill and I counted to 59 last time. A new record!"
When something is as bad as Egon's explanation of crossing the streams from Ghost Busters:
"Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light."
"Right. That's bad."