When you purchase a 12-pack of something pale and lagerlike for $6 you will almost certainly not encounter any faults that could be discerned by a professional taster. The notion of "beer faults" implies a chemical by-product of fermentation that produces a strange odor, an odor that has arisen through chemical conversion after fermentation, or a normal by-product of fermentation that is present in abnormal quantities. Beer faults fall into two main categories. First, there are faults that arise through improper brewing. These are the faults that occasionally are found in beers from small operations such as brewpubs and small craft brewers. Small breweries do not have an army of quality-control scientists to detect occasional irregularities. With some of the most explosive growth in the craft brewing revolution behind us, and a much larger cadre of more experienced professional brewers in the industry, such flaws are not as common as they once were.

Rubber (from yeast)
Cabbage, cooked vegetable (from yeast)
Medicinal aromas (undesirable fermentation by-product)
Cider (undesirable fermentation by-product)
Vinegar (by-product of bacterial action)
Sour milk (by-product of bacterial action)
Burned butter (Technical name is diacetyl, a fermentation by-product. A little is nice with ales but is frowned upon in lagers.)
Acetone (by-product of the fermentation of corn)

Condition faults: Common maladies of abused or aged beer

The second type of beer fault, much more relevant for the consumer, concerns out-of-condition beer. This is beer that apparently left the brewing vessels in tip-top condition, but due to poor handling or less than rigorous stabilization before packaging, or both, it has taken on the less noble character of age.

Light struck: Also referred to as "skunked." This flaw most commonly afflicts beers packaged in clear glass bottles. Hop oils are converted by ultraviolet light into rancid-smelling chemicals.
Oxidation: This manifests itself by telltale aromas of paper or cardboard, indicating a beer that is past its prime. It is a common problem with draft beer that has been in a half-empty barrel for too long.
Poor head formation: Head formation when a beer is poured is a property that can be controlled by a brewer, so a lackluster head is not a cause of alarm in itself. Perfectly fresh beers can form poor heads. However, when a beer is supposed to form a rich head, such as classic pilsner or Germanic lager, and fails to do so, staleness is usually the answer. A head is an emulsion of hop oil and malt proteins. If the hop oils degrade through age, the head will be proportionately poorer. (Detergent traces in the glass might equally cause lack-luster heads)

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