Nadab and Abihu took their censers, put fire in them and added
incense; and they offered unauthorized fire before
the LORD, contrary to his command.So fire came out from the presence of the LORD and
consumed them, and they died before the LORD.”
- Leviticus 10:1-2 (NIV)
Nadab and Abihu merely took fire that wasn't from the brazen alter of the tabernacle (the altar where the sacrifices were performed).
They were killed for this oversight.This is another example of how particular God is in regards to worship.