diacetyl

FACTS
produced by all yeasts and some bacteria during amino acid synthesis
reduced to a less-flavorful compound (2,3-butanediol) by yeast during conditioning

CAUSES OF HIGH LEVELS
underpitching
highly-flocculant yeast strain
inadequate diacetyl rest
bacterial contamination
high petite-mutant population

EFFECTS OF HIGH LEVELS
"butter" flavor
slick mouth-feel