query: SELECT ?item ?itemLabel ?errata ?errataLabel
  WHERE 
  {
    ?item wdt:P2507 ?errata.                      #item has a P2507
    filter not exists {?errata wdt:P921 ?item .}  #oh woe; the errata does not point back to the item
    SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }
  }