Freshman year physics, my first homework problems was a set of Fermi estimates. Ah, memories.
Then, in my 2nd semester math class, my professor told us about a time he went to buy paint for his living room. He accidentally calculated the volume instead of the surface area, and bought enough paint to cover 9,000 square feet of wall (30x30x10) instead of 1000 (30*10*4). Unrelated, until now.