After upgrading to the snapshot to test things out, I noticed that player heads were all showing as the default "Steve" skin. I respawned the heads with /give, and upon placement the game froze for about 5 seconds, before finally placing the head with the skin.

Upon attempting to place the second head, the game froze as before, but crashed a few seconds later, due to the Null Pointer Exception generated from the packet. I submitted the crash log to the hopper service, although I can also link it here.

http://pastebin.com/bZHknvM3

I did a quick search, and didn't find the bug already. If it has already been reported, sorry and feel free to close this