The bug

If a player right-clicks an animal with its spawn egg while it's playing its death animation, the game will spawn a baby animal.

Cause

According to MC-1246, a mob's hitbox doesn't instantly disappear upon death. While a mob is playing its death animation, its hitbox is still there. The game probably doesn't check if the right-clicked animal is still alive or not and therefore spawns a baby animal.

How to reproduce

1. Place a repeating and always active command block with the command /kill @e[type=pig] somewhere.
2. Spawn a pig via a spawn egg and keep the right mouse button pressed.
3. You will see a lot of baby pigs appear and instantly die until the death animation of the initially spawned adult pig ends.