Yikes, sounds like it was a close shave. Is Noodle okay?
I'd chime in on the reliability of the Herpstat, as it has options for several warning/safety settings in case of temps getting too high or too low. I get what you mean about what to do if it's an issue with the temperature probe itself, though. The Herpstats have a digital display that indicates what temp the probe is currently measuring, as well as the percentage of power output to the heat source, so I always eyeball the numbers every time I check on my snakes. I've never had this happen, but if a probe were to get dislodged, you'd be able to tell from the display readings immediately - the measured temp would be lower than target temp because the probe is pulled away from the heat source, and the power output would be climbing high in a futile effort to reach that target temp.