I have two examples from French that I think about every time a counter example comes up, which is quite frequently:
- Never say "Je suis [name]" to introduce yourself. Always say "Je m'appelle [name]". I occasionally heard counter examples of this, but I realized it really wasn't a hard rule when I listened to a podcast series in which a journalist walked along the banks of the Seine describing the area and having conversations with people. I began to keep an informal tally, and people he just walked up to were more likely to say "Je suis [name]" by quite a large margin. I still accept that in a formal situations, "Je m'appelle" is more appropriate, but the statement "French people never say Je suis to introduce themselves" is evidently wrong.
- Un autre vs encore un. Michel Thomas said that if you want another coffee you must say "Encore un café" because if you say "un autre café" it means you want a different coffee. I believe the same fact was repeated on Frenchpod101. But when I watch TV series and films and someone wants another drink in a bar or café it seems to me they almost always simply say "un autre" and they are given one more of the same drink.
What examples can you remember of things you were told that native speakers "never do" or "always do" which turned out not to be true?