Hebrews 12:15 warns that no root of what should spring up and defile many?

Answer: Bitterness

See to it that no one falls short of God's grace and that no root of bitterness springs up to cause trouble and defile many.

“See to it that no one falls short of the grace of God, and that no root of bitterness springs up to cause trouble and defile many.”— Hebrews 12:15 (BSB)