Going to the movies at a place with more than one theater, buying a ticket for one movie, and sneaking into another movie free of charge after the first movie ends.
The usher didn't notice the kids plexing, because he was looking at an attractive girl while they snuck into the second feature.
A disagreement. Usually after one person realizes something has been happening behind their back. It leaves them perPLEXed, pissed, and ready to start some shit.