Leviticus 16:1

And the Lord spake unto Moses, after the death of the two sons of Aaron,.... That is, either immediately after their death, and so this chapter would have stood in its natural order next to the tenth; or else after the above laws concerning uncleanness on various accounts were delivered out, designed to prevent the people entering into the tabernacle defiled, whereby they would have incurred the penalty of death; wherefore, as Aben Ezra observes, after the Lord had given cautions to the Israelites, that they might not die, he bid Moses to caution Aaron also, that he might not die as his sons died; these were Nadab and Abihu:

when they offered before the Lord, and died; offered strange fire, and died by flaming fire, as the Targum of Jonathan; or fire sent down from heaven, as Gersom, by lightning; see Leviticus 10:1.