unreadable DVD becomes readable - why? how?

[JohnnyBob]

I had a commercial movie disc yesterday that refused to read, stopping at about 80% with CRC errors. I tried many times with 2 different DVD drives and different software including DVD Shrink, DVD Decrypter, and DVD Fab. DVD Decrypter reported after about 150 retries that there were 2 PGC cells that were permanently unreadable. The disc had multiple tiny scratches looking like somebody had tried to wipe it clean with an abrasive cloth, but otherwise it didn't look too bad visually. I was surprised it wouldn't read. Naturally I cleaned it several times with soap and water, then window cleaner, then rubbing alcohol. Of course I restarted my computer, and reset the DMA status of my drives. Eventually I gave up on the disc as permanently unreadable. I put it aside and worked on different discs for awhile. Then before quitting my session, I decided to retry the unreadable disc one last time. Wa la, it read OK using DVD Fab... At first it gave the usual CRC error at the 80% point, then I hit the retry button, and it read OK thereafter. I tried again with DVD Fab, DVD Shrink, and DVD Decrypter and they all read it OK. No more CRC errors. I'm really mystified. How did it convert itself from unreadable to readable? It wasn't anything I did to it, as far as I know. How could a commercially pressed DVD be so unstable?

[Alex_ander]

Readability might depend on conditions of reading (like temperature), age of your drive (laser diode loses emission ability with time) etc.

[JohnnyBob]

Thanks for the suggestion but I don't think those factors are the cause. Both of my drives are fairly new and they read other discs just fine, rarely give CRC errors. There were no temperature extremes involved. Everything here is around 75 degrees Fahrenheit and the disc had plenty of time to equalize any small temperature differences.

[lordsmurf]

"New" doesn't mean anything.
A new low-quality drive will not work as reliably as an old good-quality drive.

Temperatures are relative. Extremes not needed. Affect on humidity/moisture is plenty.
I'd have to know details on your location for analysis.

Little things like the centering of the disc on the spindle can be important, too.
The drive doesn't always load a disc the same every time.

Don't be so quick to dismiss the proven science of optical media.