This Bible contradiction is from the Skeptic's Annotated Bible.
Deuteronomy 20:19 View context
When thou shalt besiege a city a long time, in making war against it to take it, thou shalt not destroy the trees thereof by forcing an axe against them: for thou mayest eat of them, and thou shalt not cut them down (for the tree of the field [is] man's [life]) to employ [them] in the siege:
2 Kings 3:19 View context
And ye shall smite every fenced city, and every choice city, and shall fell every good tree, and stop all wells of water, and mar every good piece of land with stones.