I don't know if this bug/glitch has been already reported but,
anyways I found a bug while playing Minecraft 1.6.1 release.
I were playing on Creative mode and decided to name my horse.
I renamed it as "Dinnerbone" and then it went upside down.
I tried renaming it to something else and it went normal again.

Steps to do this:
1: Get an Anvil and a Name Tag.
2. Then rename the Name Tag to "Dinnerbone" on the Anvil
3. Use the Name Tag on any nameable mob.