Starpulse: In most cases, the quality of a plot twist usually determines the quality of a movie. Bruce Willis was a ghost the whole time in The Sixth Sense. Great twist, great movie. Robert Pattinson was actually a 9/11 victim the whole time in Remember Me. Awful twist, borderline offensive movie.
But sometimes things don't add up. Sometimes, in going for that perfect little story zigzag, a film will overshoot (undershooting is also an issue) and leave us with a big twist that either makes no sense, or makes plenty of sense and is just monstrously stupid.