The Harlem Globetrotters were the only guest characters to appear three times, namely The Ghostly Creep from the Deep (1972), The Loch Ness Mess (1972), and The Mystery of Haunted Island (1973).
Dribbles, the mascot pet dog of Harlem Globe Trotters (1970), appeared in the opening theme song of this show. His only actual appearance in a story was a brief cameo in The Mystery of Haunted Island (1973).
Don Knotts was the only real life celebrity to voice himself in twice on the show. Namely, Guess Who's Knott Coming to Dinner? (1972) and The Spooky Fog of Juneberry (1972).
Last series in which Velma is voiced by Nicole Jaffe. She returned to the role for the direct-to-video movies Scooby-Doo and the Legend of the Vampire (2003) and Scooby-Doo and the Monster of Mexico (2003), and for a few short promotional skits afterward.