It was a small thing, at the time, but an anecdote I've repeated over the years is how some bullshit was printed in the political press. The senator's office (yes, it was bullshit about an actually important person, just one less important than others) told the reporter it was bullshit, and this reporter ran with it anyway.
The reason given was, basically, "my sources wouldn't lie to me because if they did I would reveal them to be liars." None of my sources would ever lie to me, because I would make them look bad, therefore everything they say is the truth. QED.
I'm sure there have been others, a few, but my only memory of a journalist outing a source as a liar is, amusingly, Howie Kurtz outing Ann Coulter.