Mr. Monk Paints His Masterpiece - S6-E14
Character mistake: While the concept of the knife height and stab wound is OK, the way Monk measures the height of the knife is way off. He says it's 48", which is 4 record lengths, but he doesn't put the bottom of the record where the top previously was, sometimes he only moves it half way up. Then the top of the record is above the knife, so it couldn't be 48" even if it was done right. As particular and accurate as Monk is, he wouldn't make this mistake.
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.