In the early days of the craft brewing boom it was entirely possible for an avid homebrewer to suddenly find themselves in the position of head brewer. With the sudden promotion came a much larger and much more complicated brew system that occasionally proved to be beyond the capabilities of the new head brewer. As a result they brewed bad beer, but not just boring tasteless trash, rather beer with identifiable faults from failures in the brewing process.

One of the most common faults was infection from a subpar sanitation at any point in the brewing or bottling process, especially if the brewery had been experimenting with sour beers. The bacteria used to produce sour beer is hard to completely eliminate and can often survive thorough sanitation efforts in scratches or other small and somewhat protected spaces like valves and couplings. To combat this many breweries use wholly separate equipment to produce sour styles. When a beer is infected it too becomes sour resulting in things like sour lagers and stouts.

Other types of bacteria occasionally infect beers as well, and some of the more common ones produce large amounts of diacetyl, a compound that tastes buttery or like rancid butter and is detectable even at low levels. Yeast also produce diacetyl, and in some cases the flavor is desirable (like oaky chardonnay), but the diacetyl and related compounds produced by bacterial infection is particularly pungent. It is often difficult for the brewer to detect as the diacetyl levels grow over time resulting in beer that tastes fine when bottled but like sour butter a month after release.

Oxidation is another fault that used to be more common. It is the result of a beer being in contact with oxygen. When beer is brewing the yeast produces CO2 which bubbles out of the beer and blankets the beer as it is heavier than O2. It also fills the head space in bottled beers. However, if a bottle isn’t properly sealed the CO2 can leak out and the beer will oxidize. This causes the beer to go flat and taste musty, mildewy, or like wet cardboard. This issue is much less common with canned beer as they are more thoroughly sealed, and less likely to leak. Oxidation can also affect individual ingredients in beer, especially the hops. Old hops that haven’t been stored properly will lose much of their character and take on off flavors if it’s particularly bad.

Beer can also be affected by exposure to strong light. The issue is called lightstruck and it’s the reason many beers come in brown bottles as they help to alleviate the issue. Canned and kegged beer is unaffected while clear and green bottles offer little protection. When a beer is lightstruck it’s hoppy characteristics break down and begin to taste skunky. This is still fairly common especially in imported beers that clear or green bottles.

Though this list is not exhaustive, these are the most common faults seen in commercially available beers. Sometimes the brewer is unaware of the issue, or it’s a rare defect that only affects a few bottles or cans at worst, but there are certainly instances of beers that have been knowingly released with these issues, or even embraced as if it were a deliberate decision. Regardless, you as a consumer should be aware of these faults and should feel empowered to demand better beer when it’s wrong.