At the climax of the Tabernacle’s dedication, the two elder sons of Aaron, Nadav and Avihu, died because they offered “a strange fire that [God] had not commanded them.” The Sages explain what exactly their sin was.

Aaron’s sons died due to four matters: for the approach, for the sacrifice, for the strange fire, and for not consulting with each other. For the approach – this is because they entered into the Holy of Holies…for the sacrifice – this is because they sacrificed an offering that they were not commanded…for the strange fire – this means that they introduced fire from a stove [not from the altar]…and for not consulting with each other –… this means that each of them acted on his own…. In four places the Torah mentions the death of Aaron’s sons, and in all of them it mentions their sin. Why is their sin emphasized to that extent? It is emphasized to inform you that they had committed only that sin. Rabbi Elazar HaModa’i said: Come and see how significant the death of Aaron’s sons is before the Holy One, blessed be He. As every place that He mentions their death in the Torah He mentions their sins. Why is their sin emphasized to that extent? It is emphasized to inform you of their sin, so that there will not be room for people to claim: They performed terrible actions in private, and that is why they died. (Vayikra Rabba 20)