Occasionally, great films are made by clever folk that suddenly go insane at some point along the way. And a twist - a narrative surprise or sudden turnaround - is a great way to ruin a movie that, up until that point, was showing enormous potential. The twist, by its very nature, is intended to flip a movie on its head, to take it in a shocking new direction that nobody saw coming.
It has to make sense as well as feel satisfying to the viewer, without dismantling everything that came before it. That's a tricky tightrope to walk; sometimes, the filmmaker just can't pull it off. And, often, the big, movie-ruining twist is saved right for the end, allowing these films a lengthy shot at greatness before they blow it all at the finish.
The inclusions below are all fairly modern - plot twists aren't a new phenomenon, but the obsession with tricking the increasingly savvy audience has grown significantly in the last decade or so, leading to just as many bad twists as good ones. Blame The Sixth Sense, as I do. Spoilers coming.