Everybody Loves a Snowman - S3-E14
Revealing mistake: As the sink with the fake tunnel is prepared, just before Newkirk goes over to set things up, you can see movement through a crack in the door. It's Schultz, Hochstetter, and Klink, waiting their cue to enter.
Everybody Loves a Snowman - S3-E14
Revealing mistake: When Hochstetter goes to open the snowman, you can see the plug is a poorly disguised piece of wood, as well as the piece of metal around the opening in the snowman.
Answer: It's a solitary cell. Steve McQueen, star of 'The Great Escape' is known as the 'Cooler King'.