When you have a Spawn Egg (Any passive mob, like a cow/sheep/pig/etc) and right click another mob of the same type, it spawns a baby version of that mob underneath it, and does NOT consume the egg.

On my server, we have a donator perk that gives out a small kit of eggs, and this bug makes it so someone can spam click a cow with a cow egg and make hundreds of animals. It was never this way before.

This was also tested while in Creative mode on a single player world on a fresh install of Minecraft.