Why Nadav and Avihu Died for Ruling Before Their Teacher

It was taught in the name of Rabbi Eliezer: Nadav and Avihu died only because they ruled on a point of law in the presence of Moses our teacher. There was once an incident concerning a certain disciple who ruled on a point of law in the presence of Rabbi Eliezer, and he said to Imma Shalom his wife: This one will not live out his week before he dies. And he did not live out his week before he died. His disciples said to him: Rabbi, are you a prophet? He said to them: I am neither a prophet nor the son of a prophet, but this is what I have received as tradition: that anyone who rules on a point of law in the presence of his teacher is liable to death. Rabbi Eliezer taught: It is forbidden for a disciple to rule on a point of law in the presence of his teacher unless he is at a distance of twelve mil from him, like the camp of Israel, as it is written, "And they encamped by the Jordan from Beth-jeshimoth to Abel-shittim" (Numbers 33:49). Like this case of Rabbi Tanchum son of Rabbi Yirmeyah, who was in Chafar, and they would ask him questions and he would rule, ask him and he would rule. They said to him: Rabbi, did you not teach us thus, that it is forbidden for a disciple to rule on a point of law in the presence of his teacher unless he is at a distance of twelve mil from him, like the camp of Israel; and behold, Rabbi Mani sits in Sepphoris! He said to them: May it come upon me if I knew. From that hour on he did not rule.