I was playing on my server, hosted on my computer. I summoned an Ender Dragon in the overworld to mess around with it in the new snapshot 14w04a, gave myself full diamond armor and switched from creative mode to survival mode.

As soon as Dragon hit me, I was kicked out from the server, which didn't crash. So I reloged and the dragon hit me again, which once again crashed kicked me out from the server. And it does that every time the dragon attacks me. It doesn't seem to happen on singleplayer worlds, because my friend tried it in singleplayer and didn't get any crash or weird behavior.

Steps to Reproduce:
1. Summon an Ender Dragon in minecraft server
2. Let the Ender Dragon hit you in survival/adventure mode
3. You are now kicked from the game and you need to reconnect