DVD Rot

We've known for years that many CDs start to oxidize and become unuseable, so it makes sense that it would be a problem with DVDs, too:

[I]t has come to light in recent months that a certain percentage of DVDs are suffering deteriorating effects,
such as cloudy areas (that look like coffee stains), holes, and specs that show up after repeated playings. In
addition, some multi-layered DVDs (DVDs that in which the movie is extremely long or has lots of features on a single
disc) seem to be experiencing layer separation or other defects that show up as skips or pixelization when the laser
in the DVD player has to switch between the layers. Sometimes the DVD player will actually freeze at this point,
preventing the playing of the next DVD layer.

The problem can mainly be chalked up to shoddy quality control at manufacturing plants, but obviously if you don't take good care of your DVDs they're eventually going to become unwatchable.