9. It Wasn't Imagined - American Psycho
There are endings on this list that were made ambiguous in a deliberate attempt to provoke thought from the audience, or leave them wanting more. Then there is American Psycho, which ended on something of a cliff-hanger almost by accident.
At the conclusion of the movie, once Patrick Bateman has confessed to his murders, he can find absolutely no trace of what he did. His lawyer took his confession as a joke, and told him that Paul Allen couldn't be dead because he just had lunch with him recently in London.
This led to speculation that the events of the movie could have been in Bateman's head all along. With no real confirmation within the movie itself, debate has raged on the subject, though director Mary Harron has put such theories to bed.
There was never meant to be any question over whether the events had really happened or not. Instead, the ending was left somewhat ambiguous as an homage to the source material. The director admitted to getting the emphasis in the final scene wrong, causing some to believe there was a question to be answered, when in reality all along Bateman got away with his murders because of his influence and reputation.