The butter flavor used in most movie theater popcorn comes from a chemical called diacetyl. It's a byproduct of the yeast used in many beers, and is considered a serious defect in most beer styles if the level is high enough to be detectable.
Interestingly, diacetyl is detectable at a level of as little as one part in ten million in a light beer. If you've ever noticed a buttery, or butterscotch flavor in a beer, that's diacetyl.
Diacetyl is added to a wide range of foods in manufacturing, and those who work in those industries are considered at risk of a serious lung disease called bronchiolitis obliterans, sometimes called "popcorn lung".
I try to avoid it.