Old prints, which are in the collections of special libraries and are the pride of Poland’s history, fall prey to thieves who are a link in the chain leading to wealthy collectors. Thieves’ practice poses a particularly great threat to Poland, which cultural heritage has been severely damaged due to historical events, especially during World War II. The purpose of this article is to present the methods and favorable internal conditions for thefts committed by employees on the example of the theft of antique books from the library in Zielona Góra. Other cases in this category of domestic theft have been described in the literature, such as the spectacular theft by internal thieves of old prints of incunabula and tzimelas in the Jagiellonian Library in Krakow detected in April 1999, or less public theft on a large scale, but interesting as a case, was the theft of antique books from the Provincial Public Library in Piotrków Trybunalski. Solving the problem of internal thefts is not favored by full inventorying and failure to perform inventories, carried out with many years of delay, as well as shortages of appropriate staff in libraries. The internal theft case described in this article shows that antiquarians are inadequately complying with their obligation to report historic items of unknown or suspected origin to the police.