I just completed one with errors. Look at the details of the completed job. Then open the purge log. Here is what I see.
The Purge operation "ElecLibPurge4" has started on 2011-06-08 10:05:40 EDT.
Attempting to purge 3347 object(s).
WARNING: Purge resulted in following errors :
Conflict Message : Branch point iterations are attempted for deletion without deleting all iterations on successive branches.
#Unable to delete following objects as they are branch point iteration.
#Type,Identity,Version,Iteration,Context
...some items listed
Attempting to purge 3337 object(s).