I guess some of you have seen this bug where enemy/fire won't go down no matter how many stimpacks or dwellers you waste. I have bumped into this bug 3 times now within short period of time and I think this is the way bug is triggered. If there is only one molerat or radroach left and last dweller in the room is near death, I assign reinforcements. Last defender dies before reinforcements reach the room and that's when that last radroach/molerat turns invincible. Same with fire, if fire is almost put out but last dweller dies before reinforcements reach the room, fire won't go out and is "invincible". What do you think?