I changed from Fullscreen mode to windowed by pressing F11 and when it changed to windowed mode my character was forced to look up and the direction i was looking kept changing very rapidly and it wouldn't stop until I pressed ESC.
By the way I am using the new Minecraft launcher and this didn't happen in the old launcher.
http://www.youtube.com/watch?v=W6FdKbC_kn0&feature=youtu.be
I recorded the issue and uploaded it to Youtube. As you can see at the beginning I am in fullscreen mode and I press F11 to go into windowed mode and this happened.

Steps to Reproduce:
1. Put the game into fullscreen mode, either by pressing F11 or the fullscreen button.
2. Once in the game press F11 to change back to windowed mode.
3. Game glitches and forces your character to look up and rapidly change direction.