I got a custom head (named Dan) and then opened up the inventory and I thought 'What the heck is the offhand slot doing?'(it had turned black and white with grey pixels flashing around.) So I then I moved the mouse onto the head (in the inventory) and the offhand slot turned back to normal. I moved the mouse onto the grey part with no slots and then the offhand slot turned all weird again.

STEPS TO REPRODUCE-

1. Type the following command- /give @p minecraft:skull 1 3

2. Open the inventory, and select three blocks to put in the first three slots on the hotbar.
3. Move Dan's head into the fourth slot, and open the survival tab.
4. Move the mouse along the hotbar and in the inventory and the shield icon should go all weird (the colours change and the texture breaks.)
(Make sure to use 1.9.1 Pre-Release 3