Unquestionably (or: undoubtedly), every competent speaker has already come to doubt with respect to the question of which form is correct or appropriate and should be used (in the standard language) when faced with two or more almost identical competing variants of words, word forms or sentence and phrase structure (e.g. German "Pizzas/Pizzen/Pizze" 'pizzas', Dutch "de drie mooiste/mooiste drie stranden" 'the three most beautiful/most beautiful three beaches', Swedish "större än jag/mig" 'taller than I/me'). Such linguistic uncertainties or "cases of doubt" (cf. i.a. Klein 2003, 2009, 2018; Müller & Szczepaniak 2017; Schmitt, Szczepaniak & Vieregge 2019; Stark 2019 as well as the useful collections of data of Duden vol. 9, Taaladvies.net, Språkriktighetsboken etc.) systematically occur also in native speakers and they do not necessarily coincide with the difficulties of second language learners. In present-day German, most grammatical uncertainties occur in the domains of inflection (nominal plural formation, genitive singular allomorphy of strong masc./neut. nouns, inflectional variation of weak masc. nouns, strong/weak adjectival inflection and comparison forms, strong/weak verb forms, perfect auxiliary selection) and word-formation (linking elements in compounds, separability of complex verbs). As for syntax, there are often doubts in connection with case choice (pseudo-partitive constructions, prepositional case government) and agreement (especially due to coordination or appositional structures). This contribution aims to present a contrastive approach to morphological and syntactic uncertainties in contemporary Germanic languages (mostly German, Dutch, and Swedish) in order to obtain a broader and more fine-grained typology of grammatical instabilities and their causes. As will be discussed, most doubts of competent speakers - a problem also for general linguistic theory - can be attributed to processes of language change in progress, to language or variety contact, to gaps and rule conflicts in ...