Friday the 13th Part V: A New Beginning
Tom Savini agreed to do the special effects for Friday the 13th Part IV: The Final Chapter because he was under the impression it was to be, well, the end of the franchise.
It was, until a year later when Paramount decided to make Friday the 13th Part V: A New Beginning, which is easily one of the worst of the series. Jason isn't even in it, for fuck's sake. Sure, someone playing adult Corey Feldman has Jason-related nightmares, and a paramedic snaps and starts pretending he is Jason, but it's a poor substitute.