Very simple to recreate. Gamemode also does not seem to matter.
Bug originally found in Multiplayer, but occurs the same in SinglePlayer.

Steps:
1. Spawn 1 Mooshroom
2. Take COW spawn eggs; right-clicking as to spawn a baby, with the cursor over the mooshroom will cause a baby Mooshroom to spawn. Not a cow as one would expect.

Thanks for your hard work!