When in a 2 block high space, you can take damage while crouching and jumping at the same time.

https://www.youtube.com/watch?v=Cg-1usLaJQE

Can confirm that it occurs when the sneak button is set to toggle instead of hold.

I was unable to recreate this in Single player, so it probably has to do with online servers. If anyone can recreate in a server, please comment.