What is a common cause of a "skunky" off-flavor in beer?

A common cause of a "skunky" off-flavor in beer is exposure to light, especially when beer is stored in clear or green bottles. This off-flavor occurs due to a chemical reaction between hops and ultraviolet light. Hops contain compounds called iso-alpha acids, which can degrade in the presence of light to form 3-methylbut-2-ene-1-thiol, a compound that has a strong skunky aroma reminiscent of a skunk's defensive spray.

The use of cans or dark brown bottles effectively minimizes exposure to light, which helps in preserving the beer's intended flavors. This protective measure is especially important for hoppy beers, which are more susceptible to light-induced off-flavors. In comparison, the other factors listed, such as excessive fermentation time, exposure to air during brewing, and improper storage temperatures, may lead to different off-flavors and spoilage but do not specifically result in the skunky flavor associated with light exposure.