This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / What Ever Happened to Baby Jane?
Jane saying happily, "You mean, all this time we could have been friends?" She was only Becoming the Mask that she was convinced of thanks to Blanche's lie. Made worse by the fact that Blanche would never have been friends with her even if she didn't try to kill her.
There is a scene where Jane reminisces about her old vaudeville act, singing her trademark song and reciting poetry with a childlike expression on her face to a Baby Jane doll. That's until she catches a glance at her reflection and breaks down in tears. Despite her behavior up to (and past) this point, you realize that, mentally, she's just a little girl that has never been allowed to grow up, is now realizing that those days are long past and doesn't know how to grow past that. She will always be Baby Jane and that's heartbreaking.