10. Halloween: The Curse Of Michael Myers
By the time the sixth Halloween movie came out the series had run out of steam in a big way. To reenergise it they introduced an origin story for Michael Myers stab happy ways with a subplot about mystic cults and rune-stones. The producers weren't happy with it during test screenings and decided it needed to move faster and be a lot gorier.
They also made the classy decision to remove most of the scenes with series star Donald Pleasance - who died shortly after filming - because they were "too boring". And so they shot a whole new finale, redid scenes with Pleasance using a stand-in and added excessive gore like an exploding head. This results in massive continuity issues and characters that seem to disappear without explanation.
With no Pleasance to rework the ending, Halloween 6 now ends abruptly with the sound of his character screaming, which implies that Myers has killed him, and then adding "In Memory of Donald Pleasance" over the credits.
The movie still sucked with the added footage but it at least gave Paul Rudd his first lead role, and he has a scene where he beats Myers to death with a pipe. So the movie isn't a total loss.