Almost had a MAJOR disaster...

[ctrlfreq]

P.S. I know people have mentioned using a back-up thermostat, but how does that work exactly? Does every strip have to be plugged into both of them??

A simple on/off thermostat will be fine as a secondary - put it in-line on the power to your main thermostat, and set the high point to a couple degrees above the acceptable range on your primary. The idea is that your primary will turn on if temps are too low, and your secondary will cut the power to the primary (and thus the flex watt) if the temperature gets too high.

You should check with the manufacturer of your thermostats in either case for correct wiring, as many have two or more power connectors that allow you to cut power to the heating elements without cutting the power required by the thermostat's circuitry.