Off-flavours in beer

Here are some informations to help you identifying unwanted flavours in beers and the causes for those off-flavours.

what described as origin comment
acetaldehyde green apple, cidery, latex paint yeast activity, microbial contamination increased by underpitching; fermentation too cool or incomplete; insufficient conditioning; bacterial infection (if accompanied by sulfur or acetic acid)
more present in young beer; might increase with oxidation
acetic sourness tart, vinegary microbial contamination increased by inoculation with bacteria or wild yeast; exposing green or packaged beer to air
high levels are caused by oxidation of ethanol by bacteria
autolyzed meaty, soy sauce, burned rubber, dirty diaper yeast, aging occurs when yeast cells die
butyric acid rancid butter, baby vomit, cheese microbes in the cellar or storage vessels never an asset
cidery green apple, apple yeast, adjunct sugars beer made with a high proportion (20% or more) of sugar often has a cidery flavor and aroma. Can also be due to weak or incomplete fermentation (see "acetaldehyde")
chlorophenol band-aid, plastic, medicinal process/equipment faults; contamination
diacetyl (Vicinal Diketones (VDK)) butter, popcorn, butterscotch, honey, milky yeast; microbial contamination natural byproducts of fermentation which should be reabsorbed by the yeast in final phases. Low levels ate acceptable in Bohemian Pilsner, English Pale and Brown Ales, some Porters and Stouts.
DMS (dimethyl sulfide) cooked vegetable, celery, cabbage, corn, sulfury malt, microbial contamination darker malts have less DMS than paler malts. Since DMS is produced at temperatures below boiling, slow cooling of the wort means that DMS is formed which isn't boiled away. Wild yeast may produce high DMS levels but also produce other off-flavors (acetic acid, phenols, sulfur).
fusel alcohols burning, alcoholic, harsh yeast avoid with proper fermentation temperature and/or proper yeast health
gushing explosive release of carbonation at the opening of the bottle; splash and overflow sugar; bacteria too much sugar used for the second fermentation in bottle or development of a mushroom from the malt; can also be caused by a high temperature change
lactic sourness citric, lactic, tart, yogurt microbial contamination caused by infection of bacteria - most commonly Lactobacillus. Medium to high levels are expected in Berliner Weisse and in sour ales.
lightstruck/skunky aroma of gaz (thiol), catty, fecal, mercaptan, skunky, sulfury mishandling decomposition of the isohumulones (compounds of hops) by the light of sun or neons; this is the reason why bottles should be brown and not clear or green to better protect the beer against this action
metallic blood-like, coppery, iron contamination caused by unprotected metals dissolving into the wort or pasteurization problems; to avoid: properly treat water to remove excessive metallic ions; don't use equipment likely to corrode
mineral chalky, dusty, sulfate, eggs water properly treat brewing water; don't use excessive amounts of brewing salts
oxidation / maderisation almond, leathery, papery, cardboard, musty aging, process faults due to wort exposed to oxygen at temperatures above 26 C; can also appear when the beer gets too old or the lagering temperature is too high