Kickin' back for a couple of minutes in a deep chair when I heard the words any host loves to hear:
"The cold water won't turn off in the guest bathroom!"
That's the original bathroom, which was installed without any shutoff valves. Okay, folks, the water to the entire house is going off...now.
Fortunately I was able to contain the gusher using old parts from the plumbing drawer in the basement. I hope that holds us until Thursday when I can get to a hardware store for repair parts from this century.