I went to the first couple of weeks of my first intro to calculus class, then just stayed home and read the book except on exam days for the rest of that semester and the other 3 classes in the sequence. The guy who taught them was great, but there were a lot of majors that required those classes, including all those offered by the business and finance schools, and so he had to cover the material very, very slowly and I was bored to tears. I found out when I had him for complex analysis years later that he gave graded homework and I probably should have failed his classes, but he had given me credit for it in my absence because he understood. Best professor ever.
That is certainly an optimized story of understanding. It's almost like taking a class from yourself; You know you haven't been to all the lectures, but your mastery of the exams was sufficient enough to elicit an "A". Then confirmed in a more engaging class years later. I've only had a few classes that bored me to tears, and the majority of the rest put me in my place proper. This fall, it's grad school for me, and I cannot WAIT to feel belittled on a regular basis once again.