Continuity mistake: When Natalie and Al are struggling, the gun lands on the table and there's a basket of corks. Somehow when Al gets the gun, the basket is tipped over and the corks are on the table. But after Monk hits Al, the corks are all back in the basket, which is upright.
Mr. Monk and the Captain's Marriage - S4-E12
Continuity mistake: While at the crime scene in the junkyard, Monk is trying to explain things, but is unable to concentrate because one of the stacked junked cars still has a headlight in it. Frustrated, the Captain shoots the light out so Monk can continue, but as the screen pulls back the headlight is still unbroken in the car.
Mr. Monk and the Other Detective - S4-E1
Continuity mistake: When the reporter is interviewing Marty's mom at the crime scene, the reporter lifts the mic to her mouth, but in the next shot the mic is away from her mouth.
Mr. Monk and the Astronaut - S4-E14
Audio problem: When the astronaut is speaking and we hear "I made a mistake, Captain. Five years ago," his lips don't match what we hear him saying. (00:11:10)
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.
Mr. Monk and the Big Reward - S4-E13
Continuity mistake: During grocery checkout, the vegetable oil keeps shifting around between shots, from next to the clerk to behind the cereal. (00:06:00 - 00:07:00)
Mr. Monk Goes to the Dentist - S4-E15
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)
Mr. Monk and Little Monk - S4-E8
Other mistake: The camera, although it is supposed to be stationary, is visibly shaking during the shot of Monk and the woman discussing the painting. (00:23:45)
Mr. Monk and Little Monk - S4-E8
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?
Answer: Notice also that in the opening credits scene when he is touching parking meters her also wipes his hand on his clothes each time he does it.