On level 6 of the Alien campaign you can actually kill everything and not finish the mission while not failing it at the same time;
You are told to make 6 Predaliens but if you make every egg into a Praetorian Egg and make all of those into Ravagers and use that horde to kill both Marines and Predators, after you kill all the Predators it won't say "All Predators Eliminated" and the mission won't end.
