Buffy The Vampire Slayer (1997)

The Prom - S3-E20

Continuity mistake: When Angel finds Buffy's notepad on the floor, it jumps from one end of the table that he puts it on to the other between shots when Joyce comes round.

THGhost

The Prom - S3-E20

Continuity mistake: When Buffy is fighting the three Hellhounds outside the school, she kills one of them and the other two enter the school, having heard some music playing from inside. But when Buffy follows them inside, the music has stopped.

THGhost

The Prom - S3-E20

Other mistake: When we first see one of the Hellhounds at the clothes store that Cordelia works at, it jumps through the window, smashing the glass, and then attacks a customer who's wearing a tuxedo. But the Hellhound attacks the customer several seconds after it smashes through the window, giving the customer a chance to escape. But the customer just stands there unaware of the Hellhound, having not heard the window being smashed.

THGhost