Mr. Monk Goes to the Carnival - S1-E5
Plot hole: It makes no sense why the police haven't done the most obvious thing and checked for fingerprints on the murder weapon (the knife). It would not only back up the detective's claim that he did not stab the victim (he never touched the knife) but would have given the police a major clue to finding and prosecuting the murderer.
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.