Scooby-Doo And The Ghoul School
Shaggy Rogers, Scooby-Doo, Scrappy-Doo, and Matches entered the castle in search of the missing Grimwood Girls and found the magic mirror in the entrance hall. Scooby-Doo saw his own evil reflection in the mirror and he ran away quickly. Shaggy thought it was just a distorted reflection, like a fun-house mirror, but when he turned his back the Mirror Monster grabbed him and took his place. Shaggy remained trapped in the mirror.
The Mirror Monster chased Scooby, but gave up after Matches burned him. He went back into his mirror and let Shaggy out.