Had an odd one with a shifter cable adjustment.
Tested at Sacramento, car ran, shifted fine, looked great. We put it in the box, the next weekend, drove to Famoso (6hrs), warmed it up and made a pass. Wouldn't shift into 1st gear. Had to adjust the cable. Ever since that day (10 odd years ago), i warm up and make sure it is hitting each gear clean, and check adjustment on the cable, in each position. Never had the problem again.
Not sure, to this day, how a cable changes adjustment when both lock nuts are tight, and no one touches the car for a week.