This Bible contradiction is from the Skeptic's Annotated Bible.
For offering a burnt offering.
1 Samuel 13:9 View context
And Saul said, Bring hither a burnt offering to me, and peace offerings. And he offered the burnt offering.
For incomplete genocide.
1 Samuel 15:2-3 View context
Thus saith the LORD of hosts, I remember [that] which Amalek did to Israel, how he laid [wait] for him in the way, when he came up from Egypt.
Now go and smite Amalek, and utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass.