At the end of a physics class the instructor described a problem for us to solve before the next meeting of the class. The problem had a stretched spring attached to a mass, and then released. I forget exactly what the problem was to solve, but it involved the motion of the mass after the release.

It seemed to me that there was something missing from what he told us, but I couldn't put my finger on it. I sat there for a couple of minutes after the bell staring off into space until I had it: The spring itself had mass that must be moved by the energy stored in the spring. The teacher had not provided any information on the mass of the spring, only the weight at the end.

I went up and asked about it and he (literally) yelled at me for not pointing it out before most of the class was gone.