After going through a seven-day ordination ceremony, Aaron and his sons were ready to take up their roles as priests. Before they started, though, Moses and Aaron went into the Tent of Meeting (the tabernacle or sanctuary).
9:23 Moses and Aaron then went into the tent of meeting. When they came out, they blessed the people; and the glory of the Lord appeared to all the people. Fire came out from the presence of the Lord and consumed the burnt offering and the fat portions on the altar. And when all the people saw it, they shouted for joy and fell facedown.
It is hard to think this would not have made an impression on people, but two of Aaron’s sons got carried away and did more than they should have, and offered ‘unauthorised fire’ to God. The response was swift: they died.
It seems harsh to us, but Moses explained it related to God’s holiness, and the need for separation from the unclean:
10:3 Among those who approach me, I will be proved holy; in the sight of all the people, I will be honored.
God went on to ban Aaron and his two remaining sons, and any future priests, from drinking alcohol whenever they went into the tent of meeting (the penalty was death). You could interpret this to suggest that Aaron’s two sons might have been drinking, and they died for their drunken (and therefore disrespectful) behaviour as much as for offering unauthorised fire. Drunken priests beware!