In New High German the following five declension classes of the noun are distinguished: 1) the feminine declension class (comprising all feminine nouns), 2) the so-called s-class (with masculine and neuter nouns), 3), the class of strong nouns (masculine and neuter nouns), 4) the mixed class (masculine and neuter nouns) and 5) the class of weak nouns (masculine nouns only). In the spoken register and sometimes even in the written register, speakers of German tend to inflect certain weak nouns according to the mixed or even to the strong class. The present paper consists of two major parts. In the first part, I give an account of how the inflection of weak nouns is treated in two dictionaries giving advice to readers being in doubt about the correct form. In the second part, I investigate possible reasons for the transition of nouns from the weak to the mixed or to the strong class. I argue that the weak declension differs considerably from all other declension classes in that the accusative and the dative of the singular are marked by a case suffix (absent in all other classes), that only in this class the genitive of the singular is not marked by the suffix -s and that only in the weak class there are two different forms for the accusative and the dative singular, depending on the syntactic context. Accordingly, speakers’ doubts about the correct inflection of weak nouns are explained by the deviance of the weak class from all other declension classes of Modern German.