In "The Chiller Diller Movie Thriller" from The Scooby-Doo Show, the scene where Milo Booth turns out the lights has a major animation error. In the scene before and after, Velma, Shaggy, Scooby-Doo and Scooby-Dum were standing at the door where Milo Booth turned off the lights. However, when he turns off the lights, all four characters have somehow disappeared. While you could have argued that Velma, Shaggy and the dogs moved, in the next shot, you can still see their shadows standing in the doorway.
Thank you to Samuel Kirkwood for coming up with this fun fact idea!