“Unauthorized fire”
Now Aaron's sons Nadab and Abihu took their censers, put fire in them and added incense, and offered unauthorized fire before the LORD, contrary to His command. So fire came out from the presence of the LORD and consumed them. (Leviticus 10:1–2 BSB)
What it sounds like today
A harsh, disproportionate punishment for a small ritual slip.
The cultural key
On the very day the priesthood begins, two of Aaron's sons improvise, offering 'strange fire' (esh zarah), incense God 'had not commanded.' Whether the fault was coals from the wrong source, the wrong time, or self-directed worship, the point is that they approached holiness on their own terms. The same fire that had just fallen from God to accept the offerings (9:24) now falls to consume the priests. The story is a stark warning that the God who draws near is holy, and that access to Him is a gift on His terms, not a human presumption.
Recovered meaning
The severity is the message: the holy is not to be handled casually or on one's own initiative. God's nearness is a mercy, but it is not tame, and worship invented at will can be as dangerous as it is presumptuous.