Mr. Monk and the 12th Man - S2-E9
Plot hole: Monk identifies the killer because of a shirt sleeve that gets ripped off... he noticed the button is sewed on in a unique manner with the threads parallel as opposed to the "standardized" crisscross pattern. The killer and Monk coincidentally use the same dry cleaner and Mrs. Ling sews buttons parallel because it's her style. Dialogue in the episode indicated she is the only one who sews buttons on like that. But I checked over 20 button up shirts here at my house and the ratio of crisscross to parallel button sewing methods is 50% of each - hardly some unique identification method.
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.