Joseph,
If someone tells me something is permissible and it isn’t, but I do it anyway, I’m over the aveirah, not the person who tells me it is permissible. *I* am responsible for my mitzvah observance. Other rishonim argued with Rashi on that comment, and I’ve seen Rabbi Hershel Schachter write that if was are certain that a rabbi is mistaken in his psak, we MUST not follow it. (Such situations would of course be very rare, and any rabbi would take very seriously any report of a frum Jew telling him that a kosher slaughterhouse was replacing cow with pig meat!)