Which two things were never to be burned in a grain offering to the LORD?

Answer: Leaven and honey

Leviticus 2:11 says no grain offering may be made with leaven, and no leaven or honey may be burned as a food offering.

“No grain offering that you present to the LORD may be made with leaven, for you are not to burn any leaven or honey as a food offering to the LORD.”— Leviticus 2:11 (BSB)