Here is a short blurb from the Denver Post.

Experienced craft brewers worry newcomers compromise on quality

If that glass of craft beer you ordered at the neighborhood pub tastes like buttered popcorn, that is not a good thing.

…

Gatza said quality problems include off flavors, oxidation and the presence of dimethyl sulfide, a sulfur compound produced during fermentation that gives a whiff of corn.

That buttered popcorn or butterscotch flavor can come from diacetyl, a compound that can creep into the brewing process if temperatures are wrong or equipment isn’t properly.