Having burned over 300+ dvd-r's - I ALWAYS try to keep dust off the dvd-r tray and try to "surgically" remove physical debris off the blank dvd-r media before it's burnt.
However, occasionally dust or whatever gets by and dvd-r gets burnt with that on it. I've noticed on those dvd's, using BeALL 1x media, that the area under the dust/debris is fainter than the other areas - basically lighter than the fully "burnt" areas. I've verified with nero that it could be read but my most important concern is longevity - can these shadowed burn areas degrade faster than the non-shadowed areas? In those cases, I've made duplicates just in case.
One other quick question: Do you think that it's possible for 1TB disks to be made available in the near future (regardless of intial pricing)? It's getting too expensive and too much of mess using 4.23 disks!