1) World War Z
OK, so World War Z isn’t an overall terrible film (in fact, it’s easily the most distinctive and interesting on this list). It does, however, feature a third act that pretty much derails the entire movie that’s gone before it, by shamelessly setting up a franchise in an all-too obvious manner, and by offering a solution to the movie’s zombie crisis that’s so stupid it kills whatever believability the film had. The worst part about it is that it could have all been so very different.
Originally, the plane that Gerry Lane took from Jerusalem at the end of the second act would land not in Wales, but in Russia – where Lane was taken as a soldier-slave and forced to fight zombies in an epic third-act battle set in Moscow.
This scene actually shot, as did scenes featuring Matthew Fox’s character taking Lane’s wife as a sexual hostage (yeah, this version was dark), but director Marc Forster decided to scrap it and go with the newly-shot Wales-set finale.
You can see snippets of the Russia sequence at the end of the theatrical cut of the movie, but don’t hold your breath for ever seeing this other, awesome-sounding final act in its entirety.