Argh!
Testing a bunch of boards with new caps. Managed to get one cap installed backwards somehow.
Testing one board which had me scratching my head while I was watching input voltage slowly dropping.
Had meter lead on input voltage test point as it went from 20 to 19... 18... 17...16.. BOOOOMMM!!!.. back to 20.
Mystery solved..
Just wish I didn't have my hand 1inch above the cap when it exploded. Nice hot electrolyte all over the palm of my hand. Fortunately, it wasn't long enough to get scalding hot.