One solid part of play testing is not playing the rules as intended but as they might be played in an attempt to break them. With Attack of the Mutants one theory I was playing out was the possibility of massive hordes of both mutants and ‘normies’.
Indeed it was possible without much effort to get really large groups together and simply cause havoc to the system. On the one hand I want to preserve immersion in the theme without having some unexplained rule come into effect. On the other hand there has to be game balance. Any balancing rule must be well grounded in the theme. In this case the solution to the massive horde rule turned out to be easy to figure out. I didn’t as much mind a room with a huge horde as I did a massive horde storming from room to room with ease. So the rule is quite simply, you can stack as many figures as will fit standing in a room but only 6 can pass through a door in either direction per turn. It is a simple answer. I seem to recall there was an issue in the original game about massive hordes. This solution is simple, adds to the theme and is a reasonable limitation.
Another issue has been determining if there are unbeatable combinations in the game. This would be something like certain figures together, aiding one another such that they always had unbeatable scores. So far I haven’t seen that. But I am a ways from just letting a few play tests confirm it. I am well aware that if I miss such a combination and players find it they will use it in expectation that something worse is coming and this formation is the smart way. Weeding these situations out is important for enjoyment of the game and to keep the tension alive until the final turn when the gate opens to let the players escape or the mutants to invade another planet!