skunked beer

Photodegradation of the hops is probably the most common cause. Non-scientifically speaking, evil sunlight comes in and busts up the hops structure and the busted up hops go on to cause other problems.

Lots of other things can cause beer to go bad as well.

Skunked beer is definitely because of the hops and sunlight reacting to form some pretty bad chemicals.

The compounds in hops that do that are stopped out of some clear bottle beers like new castle brown.
OP could be referring to how beer goes bad in a keg after using a hand pump to push it thus introducing oxygen which causes beer to go bad.