Building a movie series is a risky affair. You might wind up with one of the greatest and most ambitious cinematic experiences ever made, like that Tolkien trilogy, or you could waste tons of resources on a bloated sellout failure, like that other Tolkien trilogy. So it's no surprise that when it comes time for the big finale, some filmmakers are so scared to take chances that they wind up using the same tired old tricks to wrap it all up neatly.
Warning: This article contains spoilers for several trilogies from the past, present, and sadly, probably also the future.
Changing The Story In A Way That Ruins The Previous Movies
Did you enjoy the first two Terminator movies, about time travel shenanigans involving robot assassins? Sucks to be you, because in Terminator 3: Rise Of The Machines, it turns out that the apocalypse was always going to happen no matter what our main characters tried. That means everything Sarah and John went through in the first two movies was utterly pointless, and you are an idiot for even watching.
Speaking of retroactive incompetence, Ellen Ripley went through hell trying to save Hicks, Newt, and Bishop, only for Alien 3 to kill off all three before the movie even starts. Meaning Ripley could've just stayed home sipping martinis during all of Aliens and have saved exactly as many people -- that is, zero.
The first two X-Men movies center on the conflict between Professor X and Magneto, who have logical but incompatible views on how to protect mutants. That moral complexity was tossed straight out the window in X-Men: The Last Stand, in which Magneto goes from a fanatically mutant-protecting Holocaust survivor to a straight-up supervillain who unironically paraphrases Hitler speeches and feeds his followers into a meat grinder while calling them "pawns."