In The Flash (2023) Ezra Miller says to Gal Gadot "I know what sex is, I've just never experienced it!" This wasn't in the script, Ezra started hitting on Gal and they forgot to edit it out of the movie
Joaquin Phoenix is a Joker, a smoker and perhaps a midnight toker, actually I'm not sure. To be honest I don't even know what a "midnight toker" is supposed to be...
In the original cut of Fellowship of the Ring (2001), Peter Jackson, inspired by the parrot scene in Citizen Kane, would have the Eagles swoop in, save Gandalf, and shriek the audience back awake. This creative decision was overruled by New Line Cinema.
Originally in the special thanks section in the ending credits of "United 93" (2006), director Paul Greengrass included Osama Bin Laden. He stated "I never could have made this film if it weren't for him".