I just had a game in which, on my last draw (with three cards), activating any of them didn't do anything. Two were monsters, one was a potion. No numbers changed at all and the game did not acknowledge that I had completed the dungeon.
I also noticed that after completing a dungeon once and starting a new game, the space bar was restarting the game instead of running, even when Run was an option. I meant to try and replicate this, but that means I have to get all the way through the deck again, and I'm not that good at it. :)
This is an awesome little game though - thank you!