A 1969 animated series from Hanna-Barbera, Scooby-Doo, Where Are You kickstarted the "kids in vans solving mysteries" genre that would dominate the 1970's.
Bedlam in the Big Top (Season 1, Episode 10)Edit
The gang investigates the case of a ghost clown that is terrorizing a circus. The clown, using a gold coin, hypnotizes Scooby, Daphne, and Shaggy, making them believe they are circus performers and engaging them in deadly stunts.
Anyway, it turns out the ghost clown is the circus hypnotist.