Dirty spray nozzle. Something clogged up one of the nozzles. Once that happens you get a drip, and it becomes self-perpetuating getting worse until you go in and clean it out. Bob asked what we can do to prevent this in the future, and the answer was "It happens - you didn't do anything wrong." So the boiler is all cleaned out, tuned up, and tweaked for the winter. If we had to do it this way, better in September than say, February.
I guess I'll call Northeastern Chimney and schedule the appointment to have the flue swept. In for a dime, in for a dollar. Or rather, in for $238.50, in for another $200. If I time it right, I can scamper up the ladder with the chimney sweep and clean out my top gutter.