In the most ideal sense, there are no good or bad beers. The
     enjoyment of beer is a highly subjective and personal experience.
     However, in this very real and flawed world, various camps develop
     and embrace their favorites while denouncing all others. This is
     illustrated by "The best/worst beer in the world is...." posts.

     The best approach is to appreciate what beer is about and how to
     recognize the outstanding qualities of a fine beer (see previous
     question).

     Bad beer can be easily identified, however, when it has been damaged
     or spoiled. The two most common occurences are:

     "skunking"
          When beer has been exposed to strong light, either natural or
          artificial, certain components in hops alter and produce acrid
          flavors, AKA being "lightstruck". This is why beer should be
          bottled in brown bottles. Clear bottles offer no light
          protection and green is only slightly better. Technically, light
          of wavelengths from 550 nm and below can cause photochemical
          reactions in hop resins, resulting in a sulfury mercaptan which
          has a pronounced skunky character. 550 nm is roughly blue-green.
          Bottled beer can become lightstruck in less than one minute in
          bright sun, after a few hours in diffuse daylight, and in a few
          days under normal flourescent lighting.

     "spoiled"
          Also referred to as going "off". This is a more vague term and
          often refers to beer that has not been properly stored or
          handled allowing oxidation (a cardboard taste) or other
          off-flavors resulting from contamination, overheating, etc. As
          with any fermented beverage, alcohol can also turn to vinegar,
          imparting a sour taste to beer.