Sixty Minutes did a good story on this a few years ago.

One of Curveball's big items was about two buildings that were so close that WMD's could be moved from one to another across a driveway. The CIA had the buildings photographed from the air and found this to be impossible. this was reported to Bush who ignored it.

When US troops fought their way into the area the buildings were in they found they were just warehouses and there was a large wall between them. This wall was in place when the CIA took photos, but it was not there when Curveball had last been in Bagdad. Since the wall was built after he left he had no way of knowing that his story would be so easy to disprove.

When Curveball was confronted with this info he began to change his story until now he says he is telling the truth.