I'll be honest, sometimes I let my students make certain grammatical errors that aren't "important" or necessary to know better, at that point in their German studies. For example, in German 1, before students learn about Accusative, Dative, etc, I allow them to not change the articles of direct objects, or objects of some prepositions that come up early on. Once I teach them about direct objects, prepositions, indirect objects, etc, I don't confuse them with the other cases. That being said....