Somehow the guest had managed to flush but not allow the toilet valve to close completely. The water continued to run, filled the black tank and then dumped 100+ gallons of water on the bathroom floor.
I tried several times to duplicate the "stuck valve" and could not. It was stuck when we got home and I barely touched it and it snapped closed. I eased, twisted and pressed every which way and I cannot duplicate the failure.
We had same over flow water problem and couldn't figure out what our friends did. Couple years later, toilet valve would close but not smoothly. Got rebuilt kit and when taking apart discovered that if someone let up on valve real slow, it did not close all the way sometimes, not all the time. Tell your guests they let valve/pedal/handle snap up/shut. Do not let up lightly.