This Bible contradiction is from the Skeptic's Annotated Bible.
He was 18 years old and he reigned for 3 months.
2 Kings 24:8 View context
Jehoiachin [was] eighteen years old when he began to reign, and he reigned in Jerusalem three months. And his mother's name [was] Nehushta, the daughter of Elnathan of Jerusalem.
He was 8 years old and he reigned for 3 months and 10 days.
2 Chronicles 36:9 View context
Jehoiachin [was] eight years old when he began to reign, and he reigned three months and ten days in Jerusalem: and he did [that which was] evil in the sight of the LORD.