Smoke on the Water
At university one of my housemates had his girlfriend around in the winter and decided to get out his re-usable hand warmer and all you need to do is to immerse it in boiling water until the stuff inside it melts. He however decides to put it in a saucepan on the hob then go out for hours.
With my room being the closest to the kitchen I start to smell burning first; but not until the entire living room and kitchen had filled with a blueish smoke with no windows or doors open.
So I had to be the one to hold my breath, run in open all the windows and doors to the outside and make sure nothing was on fire. Thankfully, there was so much smoke the fire on the hob had been put out.
I should also point out I am asthmatic and had to go for a checkup and Chest X-ray following this as whatever was in the hand warmer had aggravated my lungs. Story credit: Reddit / inmate00