These are tough situations. I've uncovered outlets behind wallpaper & acoustic foam, found open splices in walls or crawl spaces that came loose, etc.
Part of it is looking competent & keeping good will with your customer. If I miss a bad receptacle or switch that I should have caught 1st time, I don't charge for my error time. I play by ear, trying to be fair to customer & me too. My kids have to eat same as his do.
That's 1 of the worst things about doing residential work. Generally, a lot more jacklegs have been involved along the way, doing anything that would make the light come on. You see some of that in commercial, but it's epidemic in residential.
I've seen receptacles with #16 wire in the wall without a box, fed by tapping hot & ground from a range outlet on 60 A fuses. I've seen a receptacle fed with good wiring methods from an existing circuit, but with doorbell wire. Open pull chain lights in wet crawl spaces with sump pumps tapped from the light. I've nearly given up being upset by it. I just fix it if they will pay me to do it and warn about what else needs fixing.