An 'exception' is not a failed wipe, and is likely to be because 're-mapped sectors' have been discovered during the erasure.
Essentially, if a drive detects that a sector is becoming faulty, that sector is 'set aside' and re-mapped to another sector from a batch kept spare for this reason The drive then has no way of accessing the original sector(s).
The erasure software detects these: the reason an 'exception' is raised is that it is possible that there is data on the original (set aside) sector, that although not accessible directly by the hard drive, may be readable using high level forensic techniques. By default (in most cases) sectors tend to be 512 bytes in size, so not a lot of data can be stored in a single one (but probably enough to store the Queen's phone number, for example!)
In these cases, the user's (recycler etc.) waste/security policies come into effect: if the drives possibly have very sensitive information on them - and hence possibly on any remapped sector - they would likely shred or otherwise mechanically dispose of the drive. If the data on the drive is not likely to be sensitive, then many will happily reuse the drive safe in the knowledge that only detailed forensic techniques could retrieve any (probably useless) 'remapped data'.
To summarise: If an area of the disk cannot be access, for any reason, no wipe can be attempted. HPA/DCO areas will also cause an exception if not removed but will not report the number of sectors affected. It is even possible that a drive can have a dual situation for these reasons where an 'Exception' can occur.
The Exception due to remapped sectors only affects CESG wiping. Under DOD rules this would be a pass.
Please remember too that when you use a license for an erasure, you are not paying for successful erasures, but rather the report generated on all completed erasures/over-writes.