DVD staining
 

I recently bought the TOS Season I box set from amazon and I noticed that most of the discs had "clouding" or staining on the data side (lighter and darker areas on the disc). I returned the set and got another with the same discolorations.

Is this normal for dual layer discs? Or are these discs defective? I tried playing one and it seems to work ok...but I don't want a set that will fail prematurely...

If I returned every disc that was discolored or clouded, i'd have no DVDs. Unfortunately, you can't really predict which DVDs will fail with any certainty, though there are DVDs made by certain companies that are seemingly more susceptible to rot due to manufacturing issues. I've yet to have a DVD fail on me, so it's all a matter of luck, I suppose... good luck. :)

It's only a cosmetic manufacturing flaw. I can't remember exactly what causes it... something like reflective aluminum layer cooling too rapidly during the bonding process.

I have DVDs that appeared "cloudy" and "stained" when I bought them new five years ago, and they still play perfectly.

aside from those crazy anchor bay ones (i think thats right) like the halloween 2 disc that somehow had the discs data disentigrate over time, i have tons of new movies with the "cloud" image on the back, so its hard to tell if that really means it wont last as long.