No need to rub it in.
The last time I designed something that came back to haunt me was back around 1984, when I undersized the transformer needed to turn on a contactor - it was fine when the contactor was on, but marginal during turn-on. Happily there was room for a slightly bigger transformer. Since then I can only recall one service return, which turned out to be due to an out-of-spec HP opto-isolator. If you do proper worst-case design, things don't fail very often ...
If you were doing it right, he wouldn't have to come back to get you to do it again. There's a line from the shoe-maker's song in Kismet - "the better my work, the worse my pay, but work can only be done one way".
I'm glad you found your niche. I was doing okay in Cambridge U.K. when my wife got her big chance, and the Netherlands has worked out much better for her than it has for me. Averaged over the two of us, it was the right career move, though I'd quite like to be doing better at the moment.