In English, various linguistic "rules" are issued by pedants, often schoolteachers. These rules are often dissociated from actual usage, and often from the established usage of great authors. They range from obsolete definitions that are easily recognized as ludicrous:
"Nauseous" does not mean nauseated, but nauseating. Hot dogs are nauseous, not people.
To injunctions against common syntax that unnecessarily trip up students and adults alike:
Do not start a sentence with a conjunction. Do not end a sentence with a preposition.
To attacks on common dialect features not present in the standard dialect:
"I don't want no more rice" is a double negative and means the person does want more rice.
Now, I'll begin teaching French in high school in the fall. As a non-native speaker, I sometimes find it hard to distinguish legitimate descriptions of usage from prescriptive pedantry. I want to avoid unwittingly propagating said pedantry, seeing that I'd be peeved if someone taught my future child things like that in English.
What are the most common examples I should watch out for in French? What might I have been told is wrong but is actually used daily and/or in classic literature or by particular users of French?