My client meant well when he duplicated his theme directory and renamed it.
Original theme directory: 'citrus_3'. 2nd theme directory 'citrus_3_backup-ejray'
However, the .info file was not renamed, nor was were the hook prefixes.
This caused Drupal to take the .info file from the backup directory instead of the desired directory and use files from the backup directory, so when the backup was deleted, Drupal went WSOD as it could not find the theme properly.
IMHO, at a minimum, Drupal should do a quick check to see if the directory name matches the .info file and if it does not, ignore that theme's directory, entirely.
Obviously, we who know about this do not keep theme backup copies in the theme directory, but it still makes sense to check.