If the Overlord cycles through his deck twice in the same dungeon level, the heroes are ejected from the dungeon and are forced to flee it.
This is from the latest FAQ. Has anyone else found this to be problematic? This means that if the OL deck is almost gone when a level starts, there is a very limited amount of turns available. This is made worse with Evil Genius in play - if there's 1 card left in OL deck and OL has a full hand when the deck is cycled, the heroes have only 10 turns (with Focus played) before they're ejected.
This can make it impossible to complete the larger dungeons (legendary areas and rumours). The FAQ doesn't specify if the deck has to cycle completely from beginning to end, which I guess it would, were it to be meant that way.