Using respectful family terms for your own family
A very common error is saying お母さん when referring to your own mother in conversation with someone outside your family. Use humble forms (母, haha) for your own family and respectful forms (お母さん, okasan) for others'.
Treating keigo as optional politeness
Keigo is not just about being polite; it encodes the social relationship between speaker, listener, and subject. Using casual speech with a teacher or employer signals a misunderstanding of the social context, not just a grammar error.
Conflating uchi-soto with simple formality
Uchi-soto is not the same as formal versus informal. You can be in an informal setting with an uchi group member and still use different language than you would with a soto person in the same room. The distinction is about group membership, not just setting.
Assuming urban and rural differences are only geographic
The urban-rural contrast in Japan involves values, family structures, and social problems, not just location. Texts about rural decline will reference aging populations and genkai shuraku; texts about urban life may address kodokushi and anonymity.
Ignoring cultural context in Interpretive tasks
When reading or listening to texts about Japanese community or family life, students sometimes miss implied meaning because they apply non-Japanese cultural assumptions. For example, a text about an elderly person living alone in a city carries specific social weight in the context of kodokushi that requires cultural knowledge to interpret fully.