Rabbi Leib Tropper, of kanno'us and EJF fame, has a blog in which he recently wrote that it is forbidden to challenge the halachic ruling of a Posek HaDor who is alive. He quotes the Yaavetz as his source, presumably referring to Teshuvos Yaavetz 1:5. It's a very long teshuvah but I can't see where the Yaavetz says such a thing (in fact, the Yaavetz very much stresses the importance of not honoring people over the perceived truth of Torah), nor do I find it easy to imagine that he could say such a thing. Rav Moshe Feinstein certainly did not believe this; his teshuvah on this topic is a must-read and you can download it here.