Vampire Bats and Scaredy Cats - S2-E2
Other mistake: When the Mystery Company find a torn paper in a coffin, it is written exo-6 and something else. They find later that exo is the first part of exotic. The way the paper was torn, you should be able to see the lower part of the missing letters.
Scared a Lot in Camelot - S1-E10
Other mistake: Fred calls out to the Black Knight ghost who sees and hears him. Yet it's Shaggy and Scooby in different armor who enter the room.
The Creepy Creature of Vulture's Claw - S3-E10
Other mistake: The whole time Professor Greer's the Mantis he's around seven feet tall. When he's unmasked he's suddenly his own size again.
