OK, the subject oversells this somewhat, but I'm still bloody stoked.
I was running out of ethernet ports on the switch where I sit to do most of my tinkering, and I remembered I had a broken 3comm switch. The power leds came on, but it didn't function. If it was something simple I might save 50 bucks. It used to get quite warm, so I opened the thing and was going to replace the electrolytics. Had new ones lined up and everything. Then I remembered: "Rule 1: Thou shalt check voltages!. " Thanks Dave.
Bloody plug pack was screwed. This has happened before, but it never dawned on me this time. A rummage around the junk box came up with two replacement candidates (with wrong connectors of course). Turns out one of those was buggered too, so I don't know what possibly functional piece of kit I threw out when that failed. Sigh.
Anyway. Thou shalt check voltages!