Continuity mistake: When Monk attempts to steal the truck at the gas station and grabs the sleeping driver's keys, he drives the truck in circles and then stops. When he stops, the truck is exactly parallel to the end of the concrete slab where the gas pumps are located. However, when the police arrive, the truck is now at a different angle with the back of the truck now being closer to the end of the concrete slab than the front. The gas station attendant would not have moved the truck himself as the police would have told him not to touch anything at the scene.
Factual error: In the flashback where Monk is explaining how the money was stolen, the principal finds the 5 dollar bill he wrote "good luck" on and holds it up for everyone to see. The bill he holds is a 5 dollar bill which was released in 2000, with the enlarged Lincoln. The flashback takes place in 1972.
Deliberate mistake: All the mail bombs are delivered in packages tied with knotted string. The USPO hasn't allowed string-tied packages since the mid-90s (they get caught in automated equipment), but since the knots were crucial to ID-ing the suspect in this episode, that fact had to be ignored.
Continuity mistake: Whilst Monk is being introduced to the long standing winner on the show a stagehand is adjusting the microphone on his jacket with both hands. As he says "Are you almost done?" the shot changes and the stagehand is just using his left arm to adjust it. The stagehand then pulls his arm away but the next shot he is adjusting the mic again and using both hands to do it. It keeps changing throughout the rest of the scene.
Continuity mistake: In the scene where the two guys are playing chess, the guy with the black hat knocks over the chess pieces after losing. He then starts scrambling up the pieces and leaving his hands near the chess board. When the shot changes to a wide shot of Monk walking away the guy suddenly has his hands on his head.
Character mistake: Season 3, episode 12, "Mr. Monk Gets Cabin Fever": When Randy Disher is having Chinese food in his apartment with his new girlfriend, he gets a fortune stating that he is going to get money from his uncle. Randy says that he doesn't have an uncle. In Season 5, episode 14, "Mr. Monk Visits a Farm", Randy inherits a farm from his uncle who appears to have committed suicide, but was actually murdered.
Continuity mistake: In this episode Randy is allergic to cats, in fact he has an allergic reaction even if a cat has been there lately but is not there now. But in Mr. Monk and the Sleeping Suspect, during the scene where Monk and the police are investigating the woman's destroyed house, a cat passes within a few feet of Randy, who doesn't sneeze or show any signs of his allergy.
Revealing mistake: Look at the stuntman who stands in for Tony Shalhoub when the balloon crashes and the basket scrapes and bumps along the ground. He is about fifty pounds lighter than Shalhoub, looks nothing at all like him and his hair looks like a Harpo Marx fright wig dyed black. He is one of the most obvious stand-ins I have ever seen.