If you quickly move your mouse up or down the screen when you are riding on a saddled tamed horse then open your inventory, your head will go past the normal limits and be upside down. This works with both survival and creative.
What I expected to happen was...
the screen would stop moving when you open your inventory, like when you are riding on a pig or saddleless horse.
What actually happened was...
The screen kept moving until the players head was upside down.
Steps to Reproduce:
1. Tame a horse.
2. put a saddle on the horse and ride it.
3. quickly move your mouse upwards then immediately open your inventory.