When you're mentally debugging a module and you have an intuition about the point of failure.
So you start mentally tracking variables, going down functions calls, moving from one class to another until finally you reach that one line of code that you feel is getting the wrong parameters.
You substitute in the 10 different variables you have been mentally tracking and find out that...
THE LINE OF CODE IS GETTING THE CORRECT PARAMETERS, AND IS FUNCTIONING AS IT SHOULD.

fuck.