It's always impressive to follow someone derive a full theory (general relativity, quantum field theory...) from the basics on the blackboard, and in the end it

It's always impressive to follow someone derive a full theory (general relativity, quantum field theory...) from the basics on the blackboard, and in the end it is off by a factor of 2 or pi, and everyone has to hunt for the place where it was misplaced - demonstrating that this was indeed a live derivation.