Not cleaning creosote from chimneys or fireplaces and heating stoves. If you've been burning a lot of wood, your stovepipe and chimney may have a heavy build up of creosote. If the creosote were to catch fire, your roof could catch fire, too. So clean out creosote before it wipes you out. Failure to provide adequate ventilation for fuel-burning heaters.
Gas heaters give off a lot of heat.
They may also give off carbon monoxide. So keep a window open an inch or more in the room in which you use an unvented heater. Keep warm, but don't risk your life doing it. Improper furnace maintenance. If your gas or oil furnace is not properly maintained, you could be in very big trouble.