A little single-board machine on my desk had been getting flaky: it would run for an hour, then drop dead, then sometimes come back if I wiggled the power lead. The classic dance. I had been treating it as a dying power supply, which is the lazy diagnosis, because it lets you order a part instead of getting the magnifier out.
When I finally looked properly, the barrel jack told the whole story. One of its through-hole legs had a dull, cracked ring around it, a textbook cold joint that had spent months flexing every time the cable moved. Power connectors take mechanical abuse that the rest of the board never sees, so this is exactly where they fail.
The fix was almost insultingly simple. Clean the pad, add a touch of flux, get the iron properly up to temperature, and reflow all four legs of the jack so the solder actually wetted the pad instead of sitting on top of it sulking. Thirty seconds of work, plus the ten minutes of squinting it took to spot it.
It has been solid since. No part ordered, no clever theory, just a joint that needed remaking. The annoying lesson, again, is that "it comes back when I wiggle it" is not a symptom to work around. It is the board telling you exactly where to point the iron.