And, me being the conflict-avoidy wanna-be-a-good-person kind of person that I am, I returned it with an apologetic note promising to try not to leave my laundry unattended for hours in the future.
BUT, even better, today the upstairs neighbors did a really simple but profoundly clever thing, which was they put a load in the washer and LEFT ALL THE NECESSARY QUARTERS sitting in the dryer quarter tray. Which meant that when I came by and found their wet clothes in the washer, it was the most natural thing in the world for me to put them in the dryer and start it before putting mine in the washer. (They even left me a thank-you note later.) I guess I never thought of this trick because I never had shared coin-op laundry in a building before. I've had coin-op not-shared, and shared not-coin-op, but only shared with people that lived in the same unit, so I could actually have conversations with them.
Anyway, hooray for indirect human-human interactions not breaking down into a morass of passive-aggressive terribleness as one might fear, and instead doing quite the opposite.