When I have a block in my inventory with a high data value (5 for example) the block will render as 2D in my inventory. I found it out in MP. See the screenshots to understand it better, because my english is not super good.
Thanks for understanding. xD

Update: as you see it did not happen for wool.

Note: I run minecraft with 2 cores CPU if that is important information.
I checked it in 14w10a 14w10b 14w10c 14w08a.
It only happend in 14w10a, b and c.

Have a nice day