I don't believe that would ever happen. A rheostat just allows you to turn down the power so it isn't heating at full capacity. It doesn't allow you to set a specific temperature. Even with a rheostat, you would need a thermometer to determine if you've achieve the temperature you would like. The attached picture shows the rheostat that came with my Kane mat. If your air temperature is 90, and you've set your thermostat for 80, the Kane mat wouldn't come on at all, nor would any other of your heating devices if they were plugged into the thermostat. When you get a thermostat, it comes with a probe. You can set that probe anywhere you would like, even on the surface of the Kane mat. Since I have two heating devices, one on the ceiling and one of the floor, I choose a spot somewhere in the middle but actually closer to the floor, because my goal is to measure the temperature where my tortoise is. My probe hangs down near the middle of his body.I read that thread, and that's where I started wondering if the thermostat or rheostat would be better. A lot of people seem to just plug into a thermostat, but that is measuring air temperature. My air temp is sometimes 90, so I don't want the heat mat to be 120!
When I first bought the rheostat, my rationale was that I wanted more of the heat to come from the top than from the bottom, so I intended to lower the bottom heat so that would happen. As it is now, I believe that at times, I need maximum power of both heat devices to achieve the temperature I would like, so I no longer feel the need for the rheostat.