This Bible contradiction is from the Skeptic's Annotated Bible.
Numbers 31:7 View context
And they warred against the Midianites, as the LORD commanded Moses; and they slew all the males.
Numbers 31:16-17 View context
Behold, these caused the children of Israel, through the counsel of Balaam, to commit trespass against the LORD in the matter of Peor, and there was a plague among the congregation of the LORD.
Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him.
Judges 6:1-2 View context
And the children of Israel did evil in the sight of the LORD: and the LORD delivered them into the hand of Midian seven years.
And the hand of Midian prevailed against Israel: [and] because of the Midianites the children of Israel made them the dens which [are] in the mountains, and caves, and strong holds.
Judges 6:5 View context
For they came up with their cattle and their tents, and they came as grasshoppers for multitude; [for] both they and their camels were without number: and they entered into the land to destroy it.