It seems to me sometimes there are just bad runs of DVDs. There are a few specific items where I've seen a whole slew of DVDs go bad (with some very specific symptoms--the discs themselves take on a 'hazy look' where the underside looks frosted with lines running through the disc that almost resemble some sort of crystilization. Here's a case where I'd be just as good taking a photo since it would do it a lot more justice than whatever I can write about it--since one can tell immediately that this isn't physical damage but some thing has happened inside the disc.
Anyway, my point really is that these things do happen but in specific runs. I've had 2-3 discs go bad from some of the early BBC runs of David Attenborough's works (for those into that sort of thing), but later they seemed to have fixed the problem. I can't imagine this as anything other than some sort of pressing plant error since with hundreds of DVDs I've only seen it in these isolated cases.
Features, Shorts, Live-Action and Direct-To-Video