In the Army at certain schools the instructors wear black ball caps which clearly identify them from other uniformed Soldiers.
During testing you run from station to station, perform a task at that station under the eye of a black hat instructor/grader.
At the end of the task performance the instructor/grader gives you a GO (pass) or NO-GO (fail).
At which point you go back to the end of the line and are retrained and retested.
Many times, the tested Soldier would perform the task at what they thought was the standard, only to hear those dreaded words.
The Army trains tasks, to a standard, under various conditions.
The standard does not change, only the conditions (rain, night, cold, mud, etc.)
My apologies for the confusion, in lapsing back to the old days, I forget not all have the same experiences.