Bugreport: Sometimes the "snake" enemy "dies" (stops moving), one of its circles becomes basically invincible, stuck there forever. Anyway, the room enemy count checks out and you can go the the next room. After changing rooms, that circle still counts as an enemy, keeping you "locked" in that room, and there's no way for you to go back to kill it. Forced to reload the game.
A suggestion would be to force kill enemies out of your room.