In the present paper, we analyzed a group of Russian nouns denoting professions and social roles. Historically, these nouns were masculine; in modern Russian, they can also be used with feminine agreement, but only nominative forms are regarded as normative (e.g. etot/eta vrač 'thisM/F doctor'). We showed that oblique case feminine forms occur naturally using the Web-as-corpus approach and conducted three experimental studies. We discovered that offline rating and online processing of such forms depends on their case. Firstly, this is a unique example of the properties of the form influencing the properties of the lexeme. Secondly, the fact that all oblique forms are regarded as marginal and that locative was found to be significantly worse than other oblique cases points to a deep connection between grammatical gender and inflectional classes and to the crucial role of affix syncretism in morphological processing. This presents a challenge for different approaches in theoretical morphology.