Though neither the users or NeXT technical representatives could explain the problem, it seems that select shipments of the disc became coated by a thin film. Since all the discs in the unreadable shipments arrived fogged while other shipments were unaffected, the defect may have occured during packaging.
Correcting the problem is easy: Users should clean the disc with a computer-screen or glass cleaner and a lint-free cloth.
A NeXT spokesperson said the problem had not been widely reported and any incidents were probably flukes.