Contradiction with 1 Samuel 16:1
This verse describes God's choice of David as king, contrasting with David's appointment of Amasa over Joab which reflects a human decision.
1 Samuel 16:1: And the LORD said unto Samuel, How long wilt thou mourn for Saul, seeing I have rejected him from reigning over Israel? fill thine horn with oil, and go, I will send thee to Jesse the Bethlehemite: for I have provided me a king among his sons.
Contradiction with 1 Samuel 15:23
Samuel declares that rebellion is like witchcraft and stubbornness as iniquity, implying that appointing a rebellious person like Amasa (who previously fought against David) contradicts divine obedience.
1 Samuel 15:23: For rebellion [is as] the sin of witchcraft, and stubbornness [is as] iniquity and idolatry. Because thou hast rejected the word of the LORD, he hath also rejected thee from [being] king. [witchcraft: Heb. divination]