So it turns out that the power adapter that has been on pretty much 24x7 under my desk for the last couple of years is susceptible to overheating and is the subject of a recall by Dell. Obviously, it isn’t plugged in any more although I would have thought that period was a pretty good soak test. Perhaps it just brings it closer to the point where it does overheat.
In any event, the recall affects Dell laptop adapters (also used with some port replicators) with either “P/N 9364U”, “P/N 7832D”, or “P/N 4983D” printed on the back.
If you have one of these adapters, you can check to see if the one you have is affected at http://www.delladapterprogram.com/ (I have two, one was affected and one wasn’t).