When you're riding a horse with saddle and look up and whilst moving your mouse you open the chat or anything with an inventory you're players view continues to move for a short amount of time (this time depends on how fast you moved your mouse....just anything that opens a GUI but doesn't pause the game. I'm not sure if this problem happens in multiplayer.

I would expect the players sight to stop moving when opening a GUI on a horse. So just like when not an a Horse..

1. Get a horse (spawn or find...it doesn't matter)
2. Put a saddle on it or the bug/glitch doesn't accrue
3. Wen on the horse move your mouse in any direction and open a GUI. (The chat works easiest.)
4. If you moved your mouse upwards and fast enough the players view should be upside down.