Continuity mistake: Just after Otis makes the ghost sounds from inside the house, it shows the gang standing together outside the house. Otis is with them outside, which is impossible, as he is inside the house pretending to be the ghost.
Continuity mistake: When Batman is talking to Scooby and the gang in the Batcave, in one shot he is missing his glove and is wearing a large ring, while in all other shots he had both gloves on.