The ending.
Now I say 1% but they do tease and foreshadow the scene/twist a few times throughout the course of The Joker 2, just in a way you may not initially get upon first viewing. I’d argue not because of how clever it is either, but because of how…it’s certainly a choice.
I know I said spoilers but again, seriously, this is the point of no return.
Arthur ends up back in Arkham, the humanity in him winning out over “The Joker”. Harley having abandoned him upon realising he’s not Joker, he’s Arthur. Except that’s not a figure of speech like you may think, neither are the other hints leading up to another inmate walking up to Arthur (one we have seen in the background throughout the movie though his face has been largely obscured until now).
The inmate tells Arthur a joke, before Arthur can answer with the punchline the inmate
stabs him to death. The inmate then
gives himself the Glasgow smile, revealing
he is the true Joker.
A brief director Q&A after the film further implied the events of the
first movie may be not what they seem (to us). That Arthur was imagining himself/confusing himself as “The Joker” but many of the actual actions of “Joker” in the first film was
this Joker that stabbed Arthur, “the true” Joker. And that one of the few instances of Arthur actually being Joker was the ending of the first movie, where he was wrongfully blamed for the actions of this Joker due to claiming responsibility that led to the trial in this one (and Harley wrongly falling for Arthur).
Also during the Q&A they teased this was a younger version of Heath Ledger’s Joker and this was somehow an origin movie for The Dark Knight’s Joker (even though this Gotham and Nolan’s don’t jive).