I know this can only happen via external editors (coughMCEditcough) but still...Minecraft should be able to handle horse/chicken/Mooshroom Mob Spawners...

Anyway, I put 3 Mob Spawners into my world using MCEdit, one of each: Horse, Mooshroom and Chicken. When I closed MCEdit and opened my world, it crashed upon loading.

Not that bad of a crash (I've got plenty of other worlds and I had just started the affected world), but still slightly annoying.

As I said, this can only be done via MCEdit or other 3rd-party editors, so there's no real 'need' for a How-to-replicate, but I'm providing one just in case it's needed:
1. Open any world.
2. Spawn in a Mooshroom, Chicken and Horse via any means (Spawn Egg, natural spawn, etc.)
3. Close world
4. Open MCEdit
5. Use a filter to create spawners out of the 3 specified mobs. The mobs will vanish and Spawners will appear in place of the mobs.
6. Save and close MCEdit
7. Open edited world and await crash.