The Physics Problem Students Keep Getting Wrong: Two Static Blocks Hung From a Spring Scale

This is a classic application of Newton's Laws. It makes a great demonstration of static force, equilibrium and Net Force.
Two masses are hung from opposite sides of a pulley. In between is a spring scale. What is the force reading on the scale?
Most people will report the answer to this physics problem wrong the first time through, thats ok, it's called learning!
This demonstration often comes up in introductory physics courses including AP Physics