Continuity mistake: When Monk and Al are sitting to drink wine, Al grabs his bottle with one hand to pour the wine and the other hand is hanging down to his side. In the next shot, as he's pouring the wine, he's holding the glass with the other hand. In the next shot, his hand is by his side again.
Continuity mistake: When the Captain and Randy are talking in the garage, in the shots over Randy's shoulder, the pad on his guitar strap is over his shoulder. In the shots from behind the captain, the pad is behind his back and can't be seen. (00:24:40)
Plot hole: When Stottlemeyer and the other cops arrive at the recycling center, the patrol car's siren turns on and off. The killer had said to Monk that if he called the cops, he'd kill Natalie. Coming to a hostage situation with the siren going would be a really bad idea. (00:37:45)
Continuity mistake: When Monk is doing the summation for the present-day case with Sherry's ex-husband sitting in the chair, at first you can see that the painting is directly behind him. However, shortly thereafter during the summation, the painting has somehow shifted to the viewer's left and is no longer directly behind him. While it's plausible that someone could have moved the painting, it's just not believable since less than a minute has passed. Besides, would Lt. Disher (he was closest) really take it upon himself to move evidence?