When I was in my 20s and 30s, I did all my own maintenance on cars and motorcycles. And for a while, on weekends, I drove 200mph down a drag strip in a car that I rebuilt every week!
But in my 60s, I find myself screwing up more than I'd like from lack of observation or forgetfulness. So over the winter, I did my yearly maintenance including taking the drivetrains apart, cleaning, and reassembling them on all my bikes, and this week the weather let me take them all out for the first rides.
When I started riding one on Thursday, it seemed a bit noisy when pedaling. It sounded chain-related, but seemed to be coming from the crank area although I couldn't see anything rubbing. So I spent part of the 30-mile ride imagining all sorts of things like bad bottom-bracket bearings, etc.
As soon as I got back and put it on the stand, it was obvious that it was very noisy, and coming from the rear derailleur -- not the front. After checking and double-checking, I finally saw that I had misrouted the chain when I put it back on! After passing over the upper pulley, I had run it under instead of over the cross-brace that's in between the upper and lower pullies. As a result, the chain was dragging over the tab and making a heck of a racket that apparently I couldn't hear well while on the bike at speed.
Luckily, I use a Wipperman Connex link, so correcting it took less than a minute. Lesson learned, but it doesn't do much for my self-confidence!