*What I expected to happen was...:
Items held in your hand/hotbar at the bottom are disabled while using a gui.

*What actually happened was...:
While in any gui including the pause menu, if you hold down on the screen where the hotbar would be ( ex. transferring a stack of items from the bottom visible rows of a chest ) then the item in that box of the hotbar is thrown (for those that dont know this is like pressing Q on the pc version).

This video shows this bug and another that is partially caused by it.
http://www.youtube.com/watch?v=kUX0Xl9WR-A

Steps to Reproduce:
1. Pause the game and hold down on the hotbar.
2. Open a stonecutter and hold down on where the hotbar would normally be.
3. Open a crafting table and hold down where the hotbar normally is.
4. Open a furnace and hold down where the hotbar normally is.

----WARNING----
I am not responsible for items lost due to this bug. (There is a bug caused by this one that can cause potential loss of items. See https://mojang.atlassian.net/browse/MCPE-3988 )