Well, not mathematicians. But I once was a physics student. We had a professor which was usually brilliant. But we had a few classes involving arithmetic: Dimen

Well, not mathematicians. But I once was a physics student. We had a professor which was usually brilliant. But we had a few classes involving arithmetic: Dimensional analysis or practical examples. He would scorn students that took out their calculator and say that any decent physicist could work it out within a few percents margin. Then he would work it out on the blackboard, and every single time he would be wrong by orders of magnitude and we all tried our best not to laugh.