One of the worst smells in the world has to be burnt microwave popcorn. It permeates everything and the stench lingers for days. One of my old roommates was notorious for burning her microwave popcorn and I just hated it. It even got into my clothes. People with allergies and/or breathing issues like asthma seem to be really reactive to this particular odor. In fact, it's so bad, I've noticed that some places of businesses and other shared spaces have taken the drastic step of banning microwave popcorn from their break rooms.
One of the problems of course is that some people put the bag in the microwave and start it and then walk away leaving it unattended. This of course is a big no no as Murphy's Law will dictate that the popcorn MUST burn in this situation and burn badly!
However, even slightly burnt popcorn tastes terrible in my opinion and I've noticed that many people do this. One of the problems seems to be that they think they can pick a precise number of minutes and seconds and pop it the same amount of time each time. This does NOT work folks!
The art of cooking really good microwave popcorn has to do with listening. Once you put the popcorn in, don't leave and just listen. As soon as you hear the popping stop, stop the microwave. If you let it go even a few seconds beyond this, it will be burnt slightly and will not taste as good.

