One day sometime after the consecration, two of Aaron’s sons, Nadab and Abihu, decided to burn some incense at an unauthorized time, so God burned them to death. Aaron and his other sons were pretty shocked, but Moses was all, I told you God would smite you if you didn’t follow the rules exactly. If I were you, I’d forgo the normal mourning rituals lest you piss It off more. God followed up that advice with a, Oh, by the way Aaron, don’t drink any booze while you are in my tent either, or else. I think had forgotten to mention that.
Later, Moses explained to his brother and nephews what parts of the the offerings the priests were supposed to eat, etc. Unfortunately, Aaron’s two other sons, Eleazar and Ithamar, managed to screw up the sin offering rituals, but Moses and Aaron managed to talk God down from smiting them too, lest God smite Itself out of priests.