The scene in Pulp Fiction where Butch kills Vincent.
I am pretty fucking sure it's actually a dream/imagined scenario by Butch, simply because when it ends, it cuts back to Butch in his car saying "that's how you're gonna beat 'em, Butch. They're just gonna keep underwstimatin' ya" as he pulls up to the apartment. But then, instead of getting to go in and grab his watch as he imagined, he instead runs into Marcellus in the middle of the street, leading to that whole thing with the rapists.
He does end up getting his watch, but after he and Marcellus part ways. Vincent never actually dies.
Except for the fact that Vincent is only there without Jules because Jules quit. And Vincent was in the toilet because he was constipated because he's a heroin addict.
If Jules hadn't quit, Marcellus would not have been there. And Butch wouldn't have known about any of those developments.
Although to back your theory up, Jules would've never left for doughnuts.