Common off flavors in homebrew explained and how to avoid bad tasting beer

My homebrew tastes like.....

"My homebrew tastes like I'm burping vinyl tubing and plastic bags"
Caused by Chlorophenol

What might have happened?
Using bleach to clean your brewing equipment or having highly chlorinated tap water where you live is most likely the culprate for homebrew with a 'plasticy' finish..particularly noticeable when you burp.

Preventative steps / options:
1) Avoid chlorinated water by using distilled water from your local grocer
2) Pre-treat chlorinated tap water by boiling for 15-20 minutes then cooling. This will evaporate chlorine and remove excess amounts from the water.
3) Pre-treat water with a Campden Tablet (potassium metabisulfite). The Campden tablets act as a catalyst for Chlorophenol removal.

"My homebrew tastes like spicy burning moonshine in my throat when I swallow"
Caused by Fusel alchohols

What might have happened?
High fermentation temperatures above 80 degrees Farenheit will often produce phenolic alcohols and other undesirable alcohols that are not pleasant to taste particularly in lighter bodied hombrew.

Preventative measure:
Ferment your hombrew in a cooler room that has a more consistent temperature (like a basement). Another option would be a temperature controlled refridgerator.

"My homebrew tastes like clovey bandages and/or eucalyptus mouthwash medicine" and I dumped it out because it was undrinkable :(
Caused by any number of Phenols

What might have happened?
Medicinal phenols that are noticeably astringent can be introduced during the brewing process through mashing or sparging at poor PH levels. Iodine based sanitizers may also contribute to phenolic off flavors in homebrew.

Preventative measure:
Use 5.2PH stabalizer and Star San sanitizer.

"My homebrew tastes like it's sour and/or has a slight vinger flavor
Caused by wild yeast or bacteria

What most likely happened?
Sour flavors are usually produced by bacteria or wild yeast. Sometimes this is intentional - as in the case of Sour ales like an Oud Bruin or Lambic with the use of brettanomyces - but for most beer this is not a desired flavor to have.

Preventative measure:
Sanitize, sanitize, and keep your wort covered during the cooling process. The environment is full of microbes like bacteria and wild yeast that could contaminate your wort as it cools down to fermentation temperature. Try keeping the kettle lid on until it cools to a temperature ready for the fermentor. Be conscious of fermentors and tubing made of soft materials, such as plastic, that may have developed crevices (scratches, port openings with gaskets/washers, etc.) where bacteria can grow, clean and sanitize these areas thoroughly.

"My homebrew tastes like a grassy hill as if it was made on a farm"
Possibly caused by old pre-crushed grain that may have developed mold or bacteria

What most likely happened?
Did you buy pre-packed and pre-crushed grain or one of those pre-made beer kits? Do you know how long and what conditioned it was stored in? You might be tasting/smelling the result of Aldehydes that have developed on your old malt. Aldehydes contribute to odors and are also often found in essential oils.

Preventative measure:
Buy fresh grain malts that have not been crushed so you can mill them yourself, or get grains that have been crushed recently (less than 8 weeks). At Barley Haven we crush our malts at time of our customer's order. If you intend on getting pre-crushed malts and using them at a later date, store them in a cool and dry place to ensure optimal freshness. Crushed malts should never be stored long term.

"My homebrew tastes and smells like blood!?!?"
This is a weird off-flavor that is not common, and gives us ample ground for vampire jokes about the beer brewer that had this tragedy occur. The metal smell and taste (often comparable to blood) is caused by high iron content

What might have happened?
Unless you're boiling in a metal vessel like a cast iron cauldron, it is highly likely your water source has an excessively high content of heavy metals.

Preventative steps / options:
1) Use a proper stainless steel kettle
2) Don't use well water. (If needed use filtered water)