This Bible contradiction is from the Skeptic's Annotated Bible.
When the earth was created.
Matthew 25:34 View context
Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world:
Sometime after the ascension of Jesus.
John 14:2 View context
In my Father's house are many mansions: if [it were] not [so], I would have told you. I go to prepare a place for you.