Discovered this when trying to specifically kill baby zombies.
Steps to reproduce:
1. In a world with no baby zombies, do /kill @e[type=Zombie] {IsBaby:1} and nothing happens.
2. Spawn 2 baby zombies into your world and do /kill @e[type=Zombie] {IsBaby:1} again.
What comes out are 2 weird results saying "(Player) fell out of the world", and "Killed (Player)" twice. Screenshot below in attachments.
Kill cannot have NBT data {...}