Thermostat set at 65 - Furnace went to 90 - possible causes?
there should be several other layers of protection from a fire, the issue is more economic. It might be a one-off event, like the thing's smarts freezing up. Maybe check the batteries. 

If in any doubt, you can put the old thermostat, set at a max cut-off temp like 80, in series with the new one to prevent a "crashed" smart thermostat from running the heat all day. The heat would only come on if BOTH thermostats agreed it should. Since the older one will be on for anything under 80, it normally does nothing and stays "on". 

If you have the opposite problem, it doesn't come on sometimes, then you put the backup thermostat in parallel with the smart one, so that either one will trigger. Then you set the analog thermostat for a fail-safe like 50 to prevent frozen pipes. 

You can put your backup thermostat out of the way, or next to the furnace; it need not be in the comfort zone to be effective.