The paper is a translation into Slovenian of the author's Italian communication to the international conference on Pietro Kandler's Codice Diplomatico Italijano, held in Rovinj/Rovigno in May 1989, and enlists the errors discovered either by critical publications of sources available to Franc Kos, or by Kos' own research results, both evident from his monumental »Gradivo za zgodovino Slovencev v srednjem veku« (Documents on the Slovenian History in the Middle Ages, 1-5, Ljubljana 1902-1928).