This Bible contradiction is from the Skeptic's Annotated Bible.
Judges 8:30 View context
And Gideon had threescore and ten sons of his body begotten: for he had many wives.
Judges 9:4-5 View context
And they gave him threescore and ten [pieces] of silver out of the house of Baal-berith, wherewith Abimelech hired vain and light persons, which followed him.
And he went unto his father's house at Ophrah, and slew his brethren the sons of Jerubbaal, [being] threescore and ten persons, upon one stone: notwithstanding yet Jotham the youngest son of Jerubbaal was left; for he hid himself.
Judges 9:23-24 View context
Then God sent an evil spirit between Abimelech and the men of Shechem; and the men of Shechem dealt treacherously with Abimelech:
That the cruelty [done] to the threescore and ten sons of Jerubbaal might come, and their blood be laid upon Abimelech their brother, which slew them; and upon the men of Shechem, which aided him in the killing of his brethren.