“Happy Days” episode "Hollywood: Part 3" started it all when the Fonz literally jumps over a shark while water skiing, infamously marking the show’s decline.
“Once Upon a Time” angered fans by introducing a six-year time jump in the final season, introducing a new curse and timeline, and retconning six seasons of storylines.
This awkward shift highlighted the show’s inability to replace Michael effectively. As a result, the show spent its last two seasons as a directionless shadow of its former self.