Screwballs
I think the person you were watching was being a bit pedantic, perhaps the added pressure of you watching him made him get a bit edgy.
If ,as you described, the candidate corrected and flew the rest of the test well then I personally would have not have insisted on a repeat.
If the candidate can debrief himself after an error then that is even better. Experience does play a huge part, you develop a sixth sense on how the check is going and a general feel for a candidate's ability.