What I expected to happen was
Your head would stop being teleported downwards once you are facing directly down

What actually happened was
My head twisted inside of itsself (ouch) and for a very brief moment, Dinnerbone didnt look upside-down anymore.

Steps to reproduce
1. Open Minecraft
2. Select a world with cheats enabled
3. Create any sort of redstone clock
4. Attach a commandblock to the output which will teleport your head down, relative to your current head position. /tp @p ~ ~ ~ ~ ~-10 will serve you good.
5. Open any inventory, including the chat.
6. Observe
7. Close the inventory so you dont die from head injuries (I will not be held responsible)