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

Verses

The Midianites

Genesis 37:36 View context

And the Midianites sold him into Egypt unto Potiphar, an officer of Pharaoh's, [and] captain of the guard.

The Ishmaelites

Genesis 37:28 View context

Then there passed by Midianites merchantmen; and they drew and lifted up Joseph out of the pit, and sold Joseph to the Ishmeelites for twenty [pieces] of silver: and they brought Joseph into Egypt.

Genesis 39:1 View context

And Joseph was brought down to Egypt; and Potiphar, an officer of Pharaoh, captain of the guard, an Egyptian, bought him of the hands of the Ishmeelites, which had brought him down thither.