While riding a horse, if you go under a 2 block high opening your head will go through the block.

Steps to reproduce:
1. Tame a horse and give it a saddle.
2. Ride the horse under a 2 block high gap.
3. Observe that you are inside a block.

Affects both creative and survival mode.

If this is a duplicate I apologize. I searched and didn't see the bug reported.