Diode polarity
I worked for a TV manufacturer in the 70s, before they were all made in China. The component auto-insertion machines (before SMT) were clever enough to check each diode polarity and rotate by 180 if it was wrong. That's fine until a batch of mis-marked diodes passed through. Of course the TVs worked fine but we'd occasionally get a repair engineer phoning us, wondering if the reversed diode was causing the fault he was chasing. It wasn't, and DON'T REVERSE THE DIODE we'd emphasise.