There's A Demon Shark In The Froggy Dark - S1-E9
Deliberate mistake: Unless they're super strong, none of Mystery Inc could've thrown an anchor to the top of a cliff.
The Scooby Doo Show (1976)
1 deliberate mistake in season 1 - chronological order
Starring: Frank Welker, Don Messick, Casey Kasem, John Stephenson
Continuity mistake: In the episode of Scooby movies where they meet the Three Stooges, Fred says that Velma is missing and they need to find her. When they go off to look for her they pass a window. Fred first, Daphne, then Velma (despite her being the one missing!), Shaggy, then Scooby.
More trivia for The Scooby Doo Show