Something is still very strange here. While overburn is generally safe and can let you squeeze a bit more data onto a CD than the 703 megs that modern "700 meg" blanks support, I have used Nero 6 since it came out (and Nero 5 before that) and I never had to use overburn to burn a Knoppix ISO (and some of the ISO have been even larger than the current ISOs). I'm still puzzled by what happened here. You mentioned that it was indeed a 703 meg disc. Was it completely virgin? If you had written a very small session on a disc and left it available for additional sessions then this would fit with the events that you document. I realize that does not seem likely, but your report of this problem is very unique.

If you do ever come across the actual source of this problem, or have similar problems burning ISOs i the future, I would appreciate you posting back.