Mr. Monk Goes Back to School - S2-E1
Corrected entry: When Monk is leaving Derek Philby's classroom, he dots the "I" in "REMINDER" on the blackboard. However, the word was in capital letters, so the "I" needn't have been dotted. Monk wouldn't make this mistake. (And the word is written in plain block sans-serif letters, so he's not adding a missing line: he's definitely dotting the capital I.)
Correction: Not all the letters in the word reminder were capitalized. It was written "ReMiNDER."