Normally, playing Minecraft in fullscreen restricts the mouse pointer to inside the game itself. However, on 1.8.2-pre5, you can move the mouse outside the usual zone, allowing you to click things in another window. When you do, however, the game minimizes (for it's not actually designed for windowed fullscreen.)

If you move the mouse out-of-bounds and back in, you cannot move it out again. You would need to close and open the GUI again for the glitch to repeat itself.

Unfortunately, I cannot get pictures of this, because Print Screen doesn't capture the mouse. I am uploading a screenshot, but I had to draw in a mouse pointer where mine was.

STEPS TO REPRODUCE:
1) Launch Minecraft, switch to fullscreen mode (if not already in fullscreen)
2) Open a world (doesn't matter whether it's singleplayer or multiplayer)
3) Open any inventory, chat, or pause the game
4) Move the mouse pointer onto a secondary monitor. You will see that it can go out-of-bounds.

SEARCH TERMS USED:
"fullscreen"
"fullscreen inventory"
"mouse"
"fullscreen mouse"