At uni I accidentally left the gas hob on for about 2 hours and sat upstairs oblivious. Came back down stairs and the smell was so intense, felt like I couldn't even see properly. Turned it off opened as many windows and doors as possible and thanked christ my smoking housemate didn't light a cig.
Could have killed at least 6 people that day, they still don't know it
Had a similar thing one Xmas with a relative whom shall remain nameless. I was sitting in the front room late at night and everyone else had gone to bed when I smelt gas and went into the kitchen to find said person leaning over the hob trying to light a cigarette (they were absolutely pissed) but was pushing one of the round knobs not the igniter. I pushed them away from the stove, turned off the gas and opened the windows while giving them the bollocking of a lifetime which woke up most of the rest of the family.