Other mistake: As Hogan and Kinchloe sit on the bunk to keep the trap door from opening, the secret switch to activate it is visible.
Continuity mistake: When Klink is "taken hostage" in the barracks, he is in his tunic only. Later in the car he has his overcoat on. He was supposed to be kidnapped, he could hardly ask Hogan to let him slip into his office for his coat.
Answer: It's a comedy, not a documentary.
stiiggy
Perhaps it was counterfeit. There are numerous episodes where they deal in counterfeit monies.