1. Placed Command Block with command

tp @p ~ ~+10 ~ ~ ~+90

2. Placed pressure plate next to command block (to power block).
3. Flew above pressure plate.
4. Opened 't' chat interface or 'e' inventory interface while falling on pressure plate.

Expected Effect: In background of interface, falling would occur and would land at tp coords.

Experienced Effect: Fell onto plate, teleported UPSIDE-DOWN (see attached image). Lands upside-down. Closing chat window (or inventory) reverses view right-side-up.

Update: In essence, this issue occurs when a window, such as the inventory or chat, is open. This issue causes a teleport with a vertical head rotation to cause the head to rotate beyond the normal limit. When the window is closed the head returns to normal rotation.

Animated GIF of issue in 15w34d: http://imgur.com/aGHwBVJ