Nobody appears to have mentioned this, so here goes:

Summoning mobs with an entity data tag to ride other mobs no longer works as of 15w41b . Tried these 3 commands in chat in singleplayer:
/summon Pig ~ ~ ~ {Riding:{id:Bat}}
/summon Skeleton ~ ~1 ~ {Riding:{id:EntityHorse}}
/summon Pig ~ ~ ~ {Riding:{Id:Bat}}
they would spawn the pig/skeleton/pig respectively not riding anything.

Note that the last one has the entity tag "Id" and not id, which is incorrect and shouldn't have worked anyway, but I tried it in case. The other two worked accurately in 15w40b. I'm guessing it was a side effect of repairing "MC-89894 – Mobs riding slimes cause the game to crash" but that's just a wild guess. Doesn't work in 15w41a either. Mobs that were already riding other mobs that have already spawned (in previous worlds) will still work. I was switching my version around and going back to a certain test world to test the bug, but I'm ~74.638% sure that shouldn't matter.