Aaron had four sons, Nadab, Abihu, Eleazar and Ithamar, who were chosen as priests to minister in the tabernacle. Nadab and Abihu had been with Moses at the foot of Mount Sinai, and they had seen the presence and glory of God. Perhaps because they were the oldest of the four sons, Nadab and Abihu were appointed to minister in the tabernacle. Their sin and their end is well known, Ex 10:1—

“Now Nadab and Abihu, the sons of Aaron, took their respective firepans, and after putting fire in them, placed incense on it and offered strange fire before the LORD, which He had not commanded them.”
“And fire came out from the presence of the LORD and consumed them, and they died before the LORD.”
“Then Moses said to Aaron, "It is what the LORD spoke, saying, 'By those who come near Me I will be treated as holy, And before all the people I will be honored.'" NASU

The sin of Nadab and Abihu was a sin of arrogance and lack of respect for the power and holiness of God. It was not just that they used strange fire, but that they did not honor God in their hearts.

Perhaps they were even drunk, considering that Moses was instructed by God about this just following their death, Lev 10:8-11—

“The LORD then spoke to Aaron, saying, ‘Do not drink wine or strong drink, neither you nor your sons with you, when you come into the tent of meeting, so that you will not die -- it is a perpetual statute throughout your generations --and so as to make a distinction between the holy and the profane, and between the unclean and the clean, and so as to teach the sons of Israel all the statutes which the LORD has spoken to them through Moses.’" NASU

Therefore we see that the real issue is honoring the power and majesty of God in our hearts. Of course what we do is a reflection of our hearts condition. If we love God, we will keep his commandments. But that does not mean that we should manufacture commandments based upon a wrong understanding of what happened to Nadab and Abihu. Many things have been identified with “strange fire” and warned against using this example, as if we are worthy of being condemned to destruction if we fail to keep certain rules and regulations invented by men.

But Aaron had two other sons, Eleazar and Ithamar, who were commanded to continue the tabernacle service without showing any remorse over what had just happened to their older brothers. But they failed to obey a specific command to eat the sin offering, Lev 10:16-20—

“But Moses searched carefully for the goat of the sin offering, and behold, it had been burned up! So he was angry with Aaron's surviving sons Eleazar and Ithamar, saying, ‘Why did you not eat the sin offering at the holy place? For it is most holy, and He gave it to you to bear away the guilt of the congregation, to make atonement for them before the LORD. Behold, since its blood had not been brought inside, into the sanctuary, you should certainly have eaten it in the sanctuary, just as I commanded.’"
“But Aaron spoke to Moses, ‘Behold, this very day they presented their sin offering and their burnt offering before the LORD. When things like these happened to me, if I had eaten a sin offering today, would it have been good in the sight of the LORD?’"
“When Moses heard that, it seemed good in his sight.” NASU

The difference between the sins of Nadab and Abihu and the sins of Eleazar and Ithamar was the difference of attitude toward God. Nadab and Abihu were arrogant and perhaps drunk. Eleazar and Ithamar were filled with sorrow over the death of their brothers.

Therefore we should learn from Aaron’s sons that the attitude of our heart as we approach God is much more important than keeping nit picking details of law.