Old customer called me out on a drawer pull mismatch

Last fall I was finishing up a kitchen install over in Arlington and the homeowner, a retired machinist named Pete, watched me put the same brushed nickel pulls on every drawer and door. He didn't say anything until I was packing up, then he just pointed at the pantry door and said, "That one's got a scratch near the screw hole, and you know it." I checked and sure enough, there was a tiny nick I'd missed in the shop. He wasn't mad, just matter of fact, like I should've caught it myself. It stuck with me because he was right, I was rushing to beat traffic and I let a flawed piece go out. I ended up swapping it the next morning for free and he tipped me 40 bucks anyway. Has anyone else had a client catch something you hoped they wouldn't notice? How do you keep yourself honest when the job is basically done?