Remnant entities (or ghosts) are assignments in tables without a corresponding entry in a
smw_proptable_hash field for an entity contains the collective hash of those values that the tables (hereby annotations) currently suppose to have assignments for to provide an efficient diffing mechanism for the updater to decided which tables require an insert/delete without reading each single table and compare its values.
The occurrence of remnant entities should rarely happen but has been observed 1 and is most likely caused by an aborted update with an unsuccessful rollback which under some circumstances can create an imbalance in the record of the
$smwgCheckForRemnantEntities2 setting is provided to help remove entities classified as remnant by running additional table queries during an update (this may have a performance impact therefore see the
DefaultSettings.php for further information).