I encountered a bug after rushing a few rooms, 4 to be exact. The first 2 succeeded but the last 2 never. The first fail was normal and worked fine but the last fail i got a mole rat attack and one of the mole rats were invincible. I sent a lot of people to go and kill it but it never worked. My dwellers were kicking and shooting the mole rat repeatedly and eventually it just killed everyone in the room so I sent another 2 rooms worth of dwellers to go and kill but it still managed to kill all of them.

This really messed up my game and I was linked here to report this bug.

EDIT: The room where it happened was 3 fully upgraded water treatment rooms.

I had the same issue, killed almost 40 dwellers. Not even an enhanced fat man could kill the last one. Mine happened in a 3 wide fully upgraded power generator. Ruined my game.

I've had this happen in two vaults now...last one was also in a 3-wide water treatment room...hmmm. Dunno if that has anything to do with it but it's an interesting coincidence. I had just splurged on a big bundle for the sale so having my vault be basically ruined an hour later is kind of frustrating.