On spawning a skeleton using the /summon command (/summon Skeleton)(completed using Tab) in chat, my world crashed.
This was done in a vanilla world while in creative and in snapshot 15w33a.
I have only tried a skeleton so far, but I will try more soon. (Reported immediately)
Edit: Also works with Zombie and Cow. I suspect all mobs have the same effect.
Steps to Reproduce:
1. Type in chat /summon Skeleton
Please update your minecraft to 15w33b, this will resolve this issue.