Mr. Monk and the Big Game - S5-E3
Stupidity: It makes no sense for the murdered coach's brother to stick around since they were getting close to him much less attend the game.
Mr. Monk Gets a New Shrink - S5-E7
Stupidity: Merrigan should've removed the heroin from the statues once he got them, thus the whole murder of the cleaning lady and discovery of his drug dealings might not have been discovered.
Mr. Monk Can't See a Thing - S5-E4
Stupidity: In the beginning, Natalie walks into the firehouse, sees two people hurt, and instead of calling 911 EMS, she calls the captain, delaying the EMS services.
Answer: The two men are trying to work out a secret deal between them without involving the union, which means the workers' interests aren't being represented and defeats the whole purpose of a union. The mayor would lose labor's support and Cusack's union troubles would just be starting.
Captain Defenestrator