Primal Fear
In the final act of Primal Fear, the young and stuttering 19-year-old Aaron (Edward Norton) is not found guilty, but a gut-wrenching twist reveals that Aaron faked his alternate personality and his speech impediment. Even his defense attorney, Martin Vail (Richard Gere), fell for the deception, which left him shaken to the core. When Vail asked if there was ever a "Roy," Aaron replies that "there was never an 'Aaron.'"
Donnie Darko
Donnie Darko is one of the most mind-bending films of all time. The plot of this film involves a number of complex timelines, events, and universes, not all of which can be condensed into a single coherent paragraph. In essence, Donnie had to die in the Tangent Universe to send the jet engine through a wormhole back to the Primary Universe. Time travel theory is extremely confusing, but there are plenty of resources to explore the topic more in-depth.