This Bible contradiction is from the Skeptic's Annotated Bible.

Verses

Abraham had only one son.

Hebrews 11:17 View context

By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten [son],

Genesis 22:2 View context

And he said, Take now thy son, thine only [son] Isaac, whom thou lovest, and get thee into the land of Moriah; and offer him there for a burnt offering upon one of the mountains which I will tell thee of.

Abraham had more than one son.

Genesis 16:15 View context

And Hagar bare Abram a son: and Abram called his son's name, which Hagar bare, Ishmael.

Genesis 21:2-3 View context

For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him.

And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac.

Genesis 25:1-2 View context

Then again Abraham took a wife, and her name [was] Keturah.

And she bare him Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah.

Galatians 4:22 View context

For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.