“The Number 23.”
The twist at the end of this Joel Shumacher dud reveals that Jim Carrey’s character was actually crazy and wrote the entire movie himself, meaning that the entire movie was pointless. However, it did teach us Jim Carrey shouldn’t ever do horror movies.
Halle Berry is the main character, a detective. It turns out that the person she is hunting is—surprise!—herself. Once you know this supposed twist, painfully re-watching the film reveals it is full of plot holes.