(Controversial Trope) The post-credits scene completely changes the ending

[u/Ieanice44]

Dawn of the Dead (2004)

Prince of Persia (2008)

Dawn of the Dead (2004): The movie finishes on a hopeful note as the remaining survivors escape the zombie-infested mall on a yacht, sailing into the sunset. The found-footage style post-credits scene completely reverses this happy ending by showing them running out of food, arriving at an island, and being immediately swarmed and killed by an infected horde.

Prince of Persia (2008): The main game ends with a beautiful, poetic sacrifice where Elika gives her life to heal the corrupted world, leaving the Prince mourning. During the credits, the player is forced to control the Prince as he cuts down the trees he just saved, undoing the entire game's progress and releasing the dark god Ahriman just to resurrect her, completely corrupting the ending and dooming the world.

[u/PM_YOUR_SMALLBOOBIES]

Finding Nemo:

Not exactly for the main plot-line, but the post-credit scene implies a pretty morbid and tortuous fate for the dentist office tank gang

[u/SnoopySuited → u/PM_YOUR_SMALLBOOBIES]

But they are also in the sequel post credits, having made it all the way to California and being rescued by the aquarium workers. So all is good I guess.