Doctor Strange In The Multiverse Of Madness: 24 WTF Moments

Unsurprisingly Sinister Strange isn't terribly trusting of the original Strange when they first meet, and so asks him for proof that he is in fact a variant.

Strange responds by revealing a deeply intimate and traumatic memory of his past, where in his youth his sister Donna drowned on an icy lake, falling in and him being unable to save her.

This is consistent with the fate of Strange's sister in the comics, and curiously enough a deleted scene from the first Doctor Strange film even depicted this.

Despite director Scott Derrickson hoping the scene would show up on the film's home video release, to date it hasn't been released.