Rather claimed to have a letter written by Bush's commanding officer claiming that during the time he was in the National Guard that he never showed up or completed his required duty training. The document, however, was on paper that did not exist in the decade in which it was supposed to have been written. IT was printed on a dot matrix printer before they existed. And it was dated after the commanding officer had died.
Even after experts discredited the letter, Rather still ran with the story. Once it was proven that it was a fake, Rather still insisted that the information in the letter was true, even though the letter itself was a fake. However he was unable to support that claim.
There is nothing to suggest that Rather was the one who faked the letter. But even with all the evidence against it, he compromised many people's trust in him when he decided to publish the story anyways. And hurt his reputation worst when he continued to insist the content was true even though the letter was fake.