I've been backing up our collection of commercial DVDs with dvdbackup, and have discovered something which seems worthy of a #PSA:

Back up your DVDs!

...because yes, DVDs do go bad (some of them), even just sitting unused. Usually you can see a lacuna or semicircular dark-streak, but sometimes its completely invisible. (I'll post some images later. update: images posted in reply)

This may have only happened to ours because they existed through a very damp summer when all the walls got mildew on them and (consequently) some of the DVDs did too (mainly those that had been opened and handled), but the mildew wipes off; this is under the plastic, so it could just be the metal substrate going bad regardless of environmental conditions.