God said “Do this” and Aaron’s sons Nadab and Abihu, priests ordained to serve the LORD in the Holy Place, did something else. We don’t know exactly what; we don’t know if it was a mistake, if they just didn’t care, if they were close to being right… but they did not follow the instructions God had given. The people of Israel are about to learn a hard lesson about God being serious about his laws and statutes, especially with the religious leaders who are responsible for his people. Shepherds are held to a higher standard because God loves his sheep.

Leviticus 10

Nadab and Abihu, the sons of Aaron, each took his censer, and put fire in them, and put incense on them, and offered strange fire before the LORD, which he had not commanded.

2 And fire went out from the LORD, and consumed them, and they died before the LORD.

3 Then Moses said to Aaron “This is what the LORD spoke of, saying “I will be sanctified in them that come near me, and before all the people I will be glorified.” And Aaron held his peace.

4 Moses called Mishael and Elzaphan, the sons of Uzziel, Aaron’s uncle, and said “Come near, carry your relatives from the front of the sanctuary outside of the camp.”

5 So they went near, and carried them in their coats out of the camp; as Moses had said.

6 Moses said to Aaron, and to Eleazar and Ithamar, his sons “Do not uncover your heads, neither tear your clothes lest you die, and wrath come on all the people: but let your brothers, the whole house of Israel, mourn those which the LORD has burned with fire.

7 You shall not go out from the door of the tent of meeting, or else you will die: for the anointing oil of the LORD is on you.” And they did according to the word of Moses.

8 And the LORD spoke to Aaron, saying:

9 “Do not drink wine nor strong drink, you, nor your sons with you, when you go into the tent of meeting, else you die: it shall be a statute forever throughout your generations:

10 That you may put a difference between holy and unholy, and between unclean and clean;

11 And that you may teach the children of Israel all the statutes which the LORD has spoken to them by the hand of Moses.”

12 Moses spoke to Aaron, and Eleazar and Ithamar, his sons that were left, “Take the grain offering that remains of the offerings of the LORD made by fire, and eat it without leaven beside the altar: for it is most holy:

13 And you shall eat it in the Holy Place, because it is your due, and your sons’ due, of the sacrifices of the LORD made by fire: for so I am commanded.

14 And the breast that is waved and the shoulder that is contributed you shall eat in a clean place; you, and your sons, and your daughters with you: for that is their due, and your sons’ due, which are given out of the sacrifices of peace offerings from the children of Israel.

15 The shoulder that is contributed and the breast that is waved they shall bring with the offerings made by fire of the fat, to wave it for a wave offering before the LORD; and it shall be yours and your sons’ with you, by a statute forever; as the LORD has commanded.”

16 And Moses inquired about the goat of the sin offering, but it was already burnt: and he was angry with Eleazar and Ithamar, the sons of Aaron which were left alive, saying:

17 “Why have you not eaten the sin offering in the Holy Place, seeing it is most holy, and God has given it to you to bear the iniquity of the congregation, to make atonement for them before the LORD?

18 The blood was not brought into the Holy Place: you should certainly have eaten it in the Holy Place, as I commanded.”

19 But Aaron answered Moses “Behold, this day they have offered their sin offering and their burnt offering before the LORD; and yet such things have happened to me: and if I had eaten the sin offering today, would it have been accepted in the sight of the LORD?”

20 And when Moses heard that, he was content.

See also: Leviticus 10 in ESV, HCSB, NASB and NIV via Bible Gateway.