Mr. Monk Meets Dale the Whale - S1-E4
Revealing mistake: When Monk is showing how Dale and his Doctor murdered the judge one can see that the knitting of the right chair is already cut to ensure that the policeman will fall through it when he later steps on it.
Mr. Monk and the Red-Headed Stranger - S1-E12
Revealing mistake: In the scene where a man is streaking, he is carefully made to look naked. But when he is running through the park, there is one shot where you can see some black underwear.
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.