Nadab and Abihu's own deaths for approaching carelessly -- the exact catastrophe this chapter's entire procedure exists to prevent from happening again.
Yahweh spoke to Moses, after the death of the two sons of Aaron, when they came near before Yahweh, and died;
Fire came out from before Yahweh, and devoured them, and they died before Yahweh.
This chapter sits at the exact structural center of Leviticus's chiasm (see the design noted on 1:1): rituals frame the book on both ends, priesthood and purity nest inside that, and the Day of Atonement holds the whole design together at its middle. Everything the rest of the book sets up - the sacrificial system, the priesthood, the purity laws - converges here, in the one ritual built to cover whatever all of it, together, still doesn't catch.