Among the VHS tapes shown to the hell-hounds to train them to attack people in formal wear at the prom is "Pump up the volume" which, despite being a high school movie, has neither a prom scene nor people in formal wear.
When the gang goes through the shop security tape, Oz asks Xander to pause, which he doesn't do for several seconds. When the tape is stopped, is shows exactly what Oz wanted to show the others, something that was not in view until then, so he couldn't have seen it when he asked for the tape to be stopped.