This Bible contradiction is from the Skeptic's Annotated Bible.
1 Samuel 15:7-8 View context
And Saul smote the Amalekites from Havilah [until] thou comest to Shur, that [is] over against Egypt.
And he took Agag the king of the Amalekites alive, and utterly destroyed all the people with the edge of the sword.
No, Agag's mother was left alive.
1 Samuel 15:33 View context
And Samuel said, As thy sword hath made women childless, so shall thy mother be childless among women. And Samuel hewed Agag in pieces before the LORD in Gilgal.
No, David killed them all a few years later.
1 Samuel 27:8-9 View context
And David and his men went up, and invaded the Geshurites, and the Gezrites, and the Amalekites: for those [nations were] of old the inhabitants of the land, as thou goest to Shur, even unto the land of Egypt.
And David smote the land, and left neither man nor woman alive, and took away the sheep, and the oxen, and the asses, and the camels, and the apparel, and returned, and came to Achish.
And then David killed them all again, just to make sure. (Except this time 400 guys escaped.)
1 Samuel 30:1 View context
And it came to pass, when David and his men were come to Ziklag on the third day, that the Amalekites had invaded the south, and Ziklag, and smitten Ziklag, and burned it with fire;
No, Saul was killed by an Amalekite.
2 Samuel 1:8-10 View context
And he said unto me, Who [art] thou? And I answered him, I [am] an Amalekite.
He said unto me again, Stand, I pray thee, upon me, and slay me: for anguish is come upon me, because my life [is] yet whole in me.
So I stood upon him, and slew him, because I was sure that he could not live after that he was fallen: and I took the crown that [was] upon his head, and the bracelet that [was] on his arm, and have brought them hither unto my lord.