The Planet of the Apes
The twist in Planet of the Apes had viewers up in arms. No one was expecting the movie to go where it did, so everything felt like it was left very open-ended. When Leo Davidson travels back in time, he finds that General Thade is already there...
Unanswered Questions
Fans have speculated as to how General Thade got to the past and took over the world. The most logical conclusion would be that Thade fixed the broken escape pod from Leo’s ship and to create his own time machine. There's no direct answer, though, but this is definitely the most accepted by the fans.