Deliberate mistake: In the opening scene, Wood's tie is grabbed by an exposed fan belt located above the side of the engine. However, this belt/pulley assembly doesn't exist in a real truck's engine compartment - the fan belts are all in front of the engine, where the camera can't see them easily. The prop master must have added this dangerous device to the truck to allow the camera to see the strangulation.
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.
Deliberate mistake: Joshua Skinner hid a fortune in gold by melting it down, mixing it with ink and writing it into 947 journals. Monk's producer concedes that in real life, 947 journals would probably hold "only about $11 worth" of gold-laced ink. But the idea was so amusing that he couldn't resist using it anyway. (00:42:00)
Deliberate mistake: As Monk is using the night vision goggles in his house the lights suddenly come back on and continues on still thinking that the intruder cannot see him. The light coming on should have blinded him or at the very least he should have been able to tell there was more light than before even if he hadn't had much experience using the goggles. All the lights in the house come back on and the amount of light would be too noticeable with night vision. This would have been deliberate as Monk continues to think the intruder still doesn't see him in the dark and is part of the joke in the scene.
Deliberate mistake: When Sharona spots Disher's personal ad in the newspaper, there is a shot of it. She then reads the description of him from the ad, which matches what's on the screen, however, the second part she reads, "Seeks petite blond with no-nonsense attitude, kids ok" is not in the ad shown. (00:22:20)
Deliberate mistake: All the dishes that are usually neatly arranged in Monk's kitchen cupboards (pots, pans, stacks of plates etc.) have been replaced here by cheap plastic ware, so that Darwin the chimp can pull them all out and throw them. Though we hear shattering-glass sound effects as they supposedly hit the floor, the dishes Darwin bangs together and throws are very obviously plastic.