Many years ago I remember seeing a little girl about 6 or 7 who was the daughter of a friend. She walked around on her toes. I asked why, they said it was a disorder (learned/self-taught behavior) they were trying to correct.
I have that (toe walking), went to a physical therapist for it and everything. nowadays it just looks weird and my shoes wear in the front instead of the heel. The doctor (rightfully) said that the passive standing on toes (for whatever reason I did that) would pass on its own as I gained weight (it did), but the gait never really changed. It did mean things like squats are more difficult, ankle flexiblity was a bit shit. It's a bit better now though, but it takes active effort to stretch and whatnot.
I don't have cerebral palsy or Duchenne, but not sure about autism :p.
I had a colleague who I'm sure did ballet or something too, he had a very upright, tight posture of sorts. I did read that women ballerinas need to (re)learn to relax their abdominals when they're pregnant to avoid certain complications.
