I have a gas furnace that seems to work fine; it keeps the house warm when things are running as expected. Sometimes, however, we'll notice the house is getting cold.
The temperature on the thermostat is set to 68., When we check the temperature reading on the thermostat, it could be as low as 64. Normally, the heater would have kicked on well before that, so we change the thermostat from "Heat" to "Off" then back to "Heat" and the heat turns on immediately.
This makes us think that it's something with the thermostat itself, but it's quite the nuisance at night. It did this a couple of times last winter, and has been happening much more this year. We've noticed no problems during the summer with it turning on the AC.