OK, so at the end of The Black Cat (1934)
Jacqueline Wells (aka Julie Bishop) goes from being fully clothed (in a nightgown) to having her legs bare and pretty risque. I have now slowed down the video to see exactly how this happened. The bottom part of her nightgown is ripped off while escaping leaving only her robe to cover her. Slowing this down shows a clear shot of her panties and legs.
Now, I know this was pre-code, but thinking about whether this was planned or not is fascinating. Just one more reason (as if you need it) to love this film.