Diacetyl can leave a slick sort of taste in the mouth or a film on the tongue
Diacetyl
Diacetyl is detected in flavor, aroma, and mouthfeel. It is often described as movie theater popcorn, buttered popcorn, buttery, toffee, buttermilk, butterscotch, or rancid butter. Oily, slick or creamy mouthfeel. Diacetyl is once again a by product of early fermentation. Generally, it is reabsorbed by yeast cells however it can be left behind by high flocculating yeast such as those originating from Britain, weak or mutated yeast, over or under oxygenating, and low fermentation temperature. Time and Temperature are the best ways to combat Diacetyl. A temperature rise near the end of fermentation, also called a diacetyl rest, provides a boost to yeast metabolism and encourages it to reabsorb diacetyl. Once again allowing your beer adequate time in contact with the yeast will allow it to clean up this fermentation byproduct.