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.

This article deals with the name of the town Węgrów on the river Liwiec, whose name is based on the obscure hydronym Węgra Potok. This name is juxtaposed with another place name, Węgra, which is found near Przasnysz and is named after the river Węgra (today known as Węgierka), as well as other names beginning with wągr-||węgr-. The author challenges Witczak’s (2015) hypothesis that the name of Węgra comes from the Sudovian (Jatvingian, Yotvingian) language. The article raises historical, archaeological and geographical arguments that oppose the possibility of a Yotvingian influence in these regions. Consequently, the author contends that the place names have a Slavonic root, linked to the noun węgorz (a type of fish), or the meandering nature of both rivers. There is also a discussion of the name patok||potok (stream/brook).