Accidents happen in beer—and it gets worse than a spilled mug. Slight, often unintentional changes in fermentation, equipment sterilization and ingredients can lend beer some undesirable tastes—referred to as “off flavors” in the brewing world. Below, we’ve highlighted ten common off flavors, but keep in mind that many taste nuances (even pleasant ones) can become off flavors when they inappropriately occur in beer styles they shouldn’t.

Acetaldehyde/acetic acid An off flavor characterized by sour, vinegar or green apple tastes. It’s a product of a bacterial contamination—either in the brewing process or your pub’s tap system. Acetaldehyde is a naturally occurring substance in plants (also, famously, the compound that causes hangovers), while acetic acid is the byproduct of fermentation.

Diacetyl Usually an off flavor, but a desired characteristic of some English ales. Identified by a butter or butterscotch flavor and a milkiness on the palate (sometimes, soapy or oily flavors are present, too). Caused by cold, interrupted or inhibited fermentation.

Dimethyl Sulfide (DMS) An off flavor. With a taste evocative of creamed corn, cabbage, tomatoes and other vegetables, DMS is derived from malt, can be detected in wort, and is reduced by a strong boil. Acceptable in moderation in some styles, such as Munich helles, American lagers and pilsners.

Isovaleric acid An off flavor introduced by old hops. The compound, which arises as hops age and lose their bitter alpha acids, produces a flavor typified by flavors of cheese, sweat or must.

Lightstruck/skunky/3-methyl-2-butene-thiol An off flavor characterized by a sulfuric, skunky aroma, typically due to a reaction between hops’ alpha acids and sulfur compounds in the presence of sunlight or electric light. Clear, green and blue bottles are most at risk for lightstruck flavors.

Metallic An off flavor with tastes similar to iron, rust, coins or blood. Its presence has a number of causes: rusty pipes or water, high malt roasting temperatures or just stale beer.

Ortho-Chlorophenol An off flavor and a phenolic compound. With a taste akin to Band-aids or bearing a medicinal quality, Ortho-Chlorophenol may appear from bacteria, but also from chlorinated water or sanitizers.

Oxidized/stale An off flavor that transpires when beer is exposed to oxygen or high temperatures, or is otherwise past its prime. Stale or oxidized flavors come in a variety of permutations—papery or cardboardlike (see Trans-2-nonenal, below), similar to rotten produce, diaperlike, leathery, sherrylike and bready are common forms.

Solventlike An off flavor identified by a sharp, acrid scent and an unpleasant burning sensation in the swallow. Usually caused by high fermentation temperatures, and sometimes, wild yeast.

Trans-2-nonenal An off flavor typified by tastes of paper or wet cardboard, usually detected in old or stale beer. Developed during aging, the compound can be thwarted by minimizing air in aging containers and bottles.