The Chumash (Shemini 10:16 and onwards) describe an argument between Moshe and Aharon.
Aharon's two sons died by the hands of Hashem and he became an Onen who cannot eat sacrifices. Moshe gave him an instruction from Hashem that in that case he must continue serving on the Mizbeach.
Aharon interpreted that instruction as applying only to the special sacrifices offered during the inauguration and not to his regularly-scheduled sacrifices. Therefore, he burned the Rosh Chodesh Musaf. Moshe Rabbeinu got upset until Aharon explained his reasoning to him. After that Moshe calmed down and admitted that he was right.
How was Aharon allowed to issue this ruling to begin with? Why isn't he considered to be one who issues a ruling in the presence of his teacher (who is liable to the death penalty at the hand of heaven)?