Solved one of my own problems and thought I'd share.
Lots of +R backups have played with absolutely zero problem in my living room Sony 5dvd changer (forget which model at the moment). Then one evening my wife ran into 5 movies in a row that would crap out intermittently during playback with obscure onscreen error messages like "DISK UNPLAYABLE" or "NO DISC".
I threw an entire battery of movies at it.... pressed DVD's played no problem.
About every other +R backup I tried would play for a few minutes then fail as above. I had resolved myself to hauling it back to Best Buy for replacement.
Anyway in preparation of it's return to the store I moved the Tivo out of my rack to get to the dvd player (Tivo on top).... and all of a sudden the locked up movie still in the drive started playing. I have since relocated the Tivo to a lower shelf and the dvd player is alone on it's own shelf. No more problems. Vibration from the Tivo's hard drive?
It sounds obvious enough, but try to keep in mind this all worked great for a solid year before this suddenly became a problem. And it's not like anything has ever moved in my rack (as the stupid amount of dust easily proves).
Totally strange. But a learning experience.