A student once arrived, disheveled and with 20 minutes left, to a midterm. I gave him a stern look and a copy of the exam. While I was grading the exam, I discovered with slight horror that he'd showed up to the first of two classes that I was teaching back to back in the same room -- he was enrolled in the second, and had arrived 30 minutes early. Horror turned to joy as I failed to find a single error on his exam. We had a good laugh as I returned his exam; he was justifiably proud and only slightly embarrassed.