This time of year, I spend a week in Denver, attending the country’s biggest beer festival: the Great American Beer Festival, or GABF. For a number of years, I’ve also been privileged to be one of the judges who help determine which beers will win gold, silver and bronze medals in 84 categories.

This year was a record year, with more than 4,420 beers from 673 breweries in 48 of the 50 states, plus Washington, D.C., and Guam.

For three days, 184 of my fellow judges and I hunker down in the basement of the hotel where the judging takes place. Our ultimate goal is to find the best of American brewing. One of the ways we accomplish this is to eliminate beers that we think are not as good as others, that have flaws we don’t find in others.

These may not even be the 10 most common defects because, as far as I know, no one has ever ranked beer defects by how often they occur. The 10 I chose are the flaws you’re likely to encounter if you drink your fair share of beer. For each one, I’ve given its common descriptor, the scientific name brewers use and its likely cause or causes.

It’s possible that you have tasted one of these defects in a beer you’ve enjoyed and didn’t realize it wasn’t supposed to be there. For some defects, it’s considered a flaw only if it’s too pronounced. At low levels, it may be perfectly acceptable — even desirable. Or it may be that you noticed one of these off-flavors but didn’t know its cause. Some causes are quite complicated, so I’ve tried to simplify them as best I could. Hopefully, this will give you a better sense of some of the more unusual flavors you sometimes find in a beer but wished you didn’t.

The defects

Band-Aid (chlorophenol): You just know something is out of whack when your beer smells of adhesive bandages. This aroma also may remind you of disinfectant or diaper aromas. It’s the artificial quality that really stands out in this defect, which usually is caused by a problem with sanitizers or yeast.

Butter or butterscotch (diacetyl): Think of that artificial butter aroma and flavor from movie-theater popcorn, and you’ve got the diacetyl character in some beers. At low levels, this can be an enjoyable flavor component. But just like popcorn that’s swimming in butter, too much can make for an unpleasant experience. Beers with too much diacetyl often are called “butter bombs,” and the cause is often a problem with the yeast and amino acids.

Cardboard (oxidized): The aroma usually will remind you of wet cardboard or wet paper, like if you left a box out in the rain and then took a whiff of it. Sometimes, it may seem leathery. It can be a sign of boiling too long, but more often it’s simply stale beer that’s too old or been stored improperly.

Cheesy (isovaleric acid): If you get a whiff of bad cheese or stinky feet, use your own to run away. It’s a doozy. It can have a benign origin, such as the poor storage of hops, or it could be a bacterial infection.

Cooked corn or cabbage (dimethylsulfide): Often called DMS, if this is in your beer’s nose, it’s probably a sign of something gone awry, especially in ales. It also may have an asparagus or vegetal smell. In dark beer, the aroma may remind you of tomato soup. Its cause is commonly a grain infection or brewhouse problem, which usually occurs﻿ in the boil.

Green apples (acetaldehyde): If you smell green apples or green leaves, it’s most likely a sign that the beer was released too soon, or that there was a yeast metabolic problem. Like its aroma, the beer is a little green. While usually evidence of a defect, it’s not as unpleasant a problem as many others.

Lightstruck (methyl mercaptan or isopentyl mercaptan): A beer can become lightstruck, causing it to smell like a skunk; it happens almost instantaneously when it encounters light, especially UV rays. Fluorescent lights and bright sunlight are common culprits. Since both clear and green glass offer much less protection, many popular brands of beer are very susceptible to this problem. Brown glass, while not perfect, offers the most protection of any common glass color, which is why most brewers use this for their bottles.

Metallic (lacquerlike, metallic): It’s important to note that this would hardly ever come from canned beer. The metal turbidity that once caused metallic flavors to leach into canned beer has been virtually eliminated. Today, can manufacturers spray an organic polymer inside the can so that the beer literally never touches the aluminum. Metal flavors in beer are usually bitter, and they’re always bad, caused primarily by iron, copper or other metals in the water.

Plastic (phenolic): The aromas in your beer should never seem artificial, and that’s what phenolics smell like. They have an artificial aroma that can take the form of something medicinal, mouthwash or plastic, and they’re cause is often a problem with the water, yeast or sparging