IMPORTANT TO NOTE: This is not my (actual) world view (opinion) of anyone really dealing with a multiple personality disorder...
...but I suspect this person wasn't really dealing with a MPD.
“Oh, I'm so sorry! (I just bit your head off) Must be because of my multiple personalities.”
Hmmm...(thinks to myself) I've met this (bus stop) mom several times and since I have ONLY met this, snipey, personality I doubt it's a “multiple” problem but in fact, a problem because it is the ONLY personality she has, so (trying to be helpful) I say...
“It must be a challenge dealing with that but, perhaps, you could just tag the nice one before you head out the door to interact with people.”
Guess what? She wasn't able to do that. Hahahahaha!