When the inventory is open while you are flying with an elytra, the player model in the inventory appear very weird and sometimes upside-down and/or outside of the boundaries. The player in the inventory also follows wherever the mouse curser is, and rotates when facing different ways in the world. 

I also searched for this in the bug tracker, and I found someone else mentioning this back in 1.9. On there it said it was resolved, even though it isn't.