One of the planned Joker endings never happened because Christopher Nolan reportedly vetoed it.
Joker 2's controversial ending was originally held back by Dark Knight director Christopher Nolan
According to a report in The Hollywood Reporter, Joker was to have an ending where Joaquin Phoenix's Arthur Fleck sliced his face into a smile in front of a crowd of supporters. However, Nolan stepped in and "killed that idea." Nolan believed that only his Joker, played by Heath Ledger in The Dark Knight, would need to slice his face.
For Joker: Folie à Deux, Phillips added a scene where someone else cuts his face. At the end of the film, the patient Arthur stabs starts cutting his face in the background while laughing hysterically.
Nolan left Warner Bros. for Universal to cast Oppenheimer, meaning the Oscar winner was no longer around to cut the scene. Furthermore, the success of Joker led to Phillips being given an "extraordinary level of autonomy and definitive cut" for Folie à Deux.
