Contradiction with 2 Kings 16:2
It states that Ahaz was 20 years old when he began to reign, which creates a chronological issue if Hezekiah, his son, began reigning at 25 years old right after him.
2 Kings 16:2: Twenty years old [was] Ahaz when he began to reign, and reigned sixteen years in Jerusalem, and did not [that which was] right in the sight of the LORD his God, like David his father.
Contradiction with 2 Chronicles 29:1
It mentions Hezekiah also became king when he was 25 years old, indirectly supporting 2 Kings 18:1, but not providing a direct contradiction; contradictions generally involve misinterpretations or consistencies around the reigns and timelines.
2 Chronicles 29:1: Hezekiah began to reign [when he was] five and twenty years old, and he reigned nine and twenty years in Jerusalem. And his mother's name [was] Abijah, the daughter of Zechariah.