Customer told me my CLA work was 'sloppy' and I actually listened
Old guy brought in a Pentax Spotmatic he'd had since the 70s for a CLA. I did my usual thing, it looked fine to me. He called me the next day, came back, and showed me a tiny spec of dust on the mirror and said the film advance felt 'gritty'. I blew him off at first, but then I re-checked it under a strong light and found like 3 spots I missed. Now I use a magnifier and a rocket blower on every single mirror and lens element before I seal it back up. What's the one thing a customer pointed out that made you change your routine for good?