I don't know why, but I'm really enjoying these double contractions ("Mustn't've," "I'm'a", etc.). Or, perhaps just that "double contractions" exist at all in the English language. It seems more fun than boring old regular contractions.
(If I was a tone-deaf corporate social media account, I'd end with something like "post your favorite double contraction stories below!" But I'm not. I don't want to hear your double contraction stories. Keep'm't'y'rs'lf.)