When I was in college, I noticed that Tom Lehrer was teaching a math course "Nature of Math" and took it. Great intro to all sorts of historical details like how people competed to solve quartic equations in the 1500s, etc, all delivered with that special Tom Lehrer joking attitude. It was also my introduction to the birthday paradox, which was presented entirely without reference to hash tables.

After I finished, I had to decide between TAing that course, or working on an undergraduate thesis; I did the latter, which helped launch my career, but I regret not working with Tom.