You wouldn't call out a mistake? That's bad, especially for newer playerss and genuine mistakes. If you don't tell them that it's not actually the rules, they won't know, and could as easily get into trouble later for not knowing the rules. If someone playing against me makes what I'd call a fair mistake, that is, a one-off, an easy misinterpretation, or a errant rule they apply to both sides, then I'll just tell them that it's wrong, and back it up with the main rules. If, though, they persist in doing it, or are using different rules that quite clearly convey an advantage to them only, I'll point it out much more visibly, and be prepeared for an argument.