It Was His Sled
A Twist Ending that used to be guarded carefully as a Spoiler, but thanks to Popcultural Osmosis, everyone within the target demographic knows the ending already, even those who haven't even seen the show. It's probably never going to surprise anyone again. In many cases, the twist becomes the central fact known even to those only noddingly familiar with the work, and other adaptations take it as read from the beginning.
Named after a line from Family Guy where Peter records the above quote over the best-known example of this trope, Orson Welles' film Citizen Kane.
Sometimes the work will be remade/adapted, and the ones responsible will know better. That's when they twist the twist itself, making it Not His Sled.
- Dragon Ball: Exactly what entailed being a Super Saiyan was a mystery at one point, but it's become such an iconic part of the series that you almost certainly know about it by now even if you're not a Dragon Ball fan.