Anal Annuals
Doing annual inspections for over 15 years I've learned that pulling the plugs for cleaning and re-gapping tends to break loose the lead deposits that are bridged between the plug threads and the combustion chamber. I would install the plugs back in the cylinders and a lot of times those loose pieces would find their way back into a lower plug and foul it out as soon as the owner tries to taxi away after a fresh annual. Kinda makes me look bad. Now I always put each cylinder on TDC with both valves closed, take a round wire pipe cleaner brush in a drill motor in each spark plug hole to break the stuff loose then blow through the plugs holes with high pressure air. I'll usually get a palm full of stuff out of four cylinders. Since I started doing this, I've never had one foul.