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.