I was crafting a bunch of minecarts and hoppers and ended up with not enough room in my inventory, there was iron left over which now appears to be permanently attached to my mouse position whenever I open my inventory. 

1. I can't place the stack in my inventory
2. I can't pick up items from my inventory
3. I can shift+click to move items between inventories however
4. If I rapidly click in a spare inventory slot, it appears to briefly remove one item from a stack contained in my chest or inventory and then return it and also briefly drops the stack attached to the mouse position in the inventory and then returns it to the mouse
5. Exiting and reloading does detach the stack from the mouse, the total iron in my inventory does appear to be correct.