Plot hole: Hogan's men have that clever device in Klink's quarters, where they turn the oven from below to reveal the trap door. That however unplugs the oven from the chimney, you can even see the smoke pouring out of the oven in some shots. Often enough, they have the oven unplugged for half a minute or more. If an oven smoked into a room that long, the whole room would fill with smoke so badly it could actually lead to smoke poisoning. If nothing else, Klink would have the oven checked. Any device that can turn a heavy cast-iron stove flawlessly like that would by definition have to be quite massive and not disassemble easily. If Klink had the oven checked, that would certainly lead to the discovery of the turning mechanism.