In the paper, on the basis of several dozen examples of hydronyms from eastern and north-eastern Mazovia, the erroneous attribution of written sources and, as a consequence, their incorrect identification and location were shown. These kinds of mistakes can be found in toponymical, hydrographical and historical publications. It is the result of taking attestations from the context of the source, poor knowledge of the history and the geography of the studied area, as well as the poor knowledge of Latin and the lack of verification of hypotheses found in older literature.

The hydronym Szywra refers to the small river in the Warta basin flowing in the central part of Greater Poland. Although its name remained unclear for most of the researchers, it was believed to be of Pre-Slavic or Balto-Slavic origin. The paper reveals that these hypotheses were based on the wrong interpretation of the source material, and provides a new etymology for the name Szywra. Based on the critical analysis of all of the reachable records of names referring to the river Szywra, it has been proven that its Polish name is an adaptation of the former German name Schieferbach. Such a process was possible due to the long-term bilingual situation in the region of Greater Poland.