When Diana, Princess of Wales, died, I was the foreign correspondent for The Sunday Times in Paris. Our office was inundated with calls from people claiming to have witnessed the crash. One man said that he had been in the car in front of Diana's and had seen the whole thing in his rear-view mirror. He said the car had crashed because a motorbike had swerved in front of it. He sounded pretty convincing, so we put him and his wife up in a hotel and drove to the tunnel with him several times, getting him to tell us what he'd seen. We structured a lot of our story about how the accident might have happened around what he told us - including some very complicated and expensive graphics.
But I still had a niggling feeling about him. I asked a French friend who had good connections with the police to check this guy out. We were going to press when my friend rang me up and said: "Your witness is a professional witness. The police aren't doing anything because he has claimed to have been present at too many accidents."
So I rang the paper and said: "Quick - we've got to ditch it," and they said: "We can't - the graphics are done." So I had to rewrite the story in a way that still used what the man had said but also made it clear that we didn't believe him. It wrecked the story and I felt acutely embarrassed about the whole thing.
Then, about a year later, ITV ran a documentary alleging that Diana had been murdered. I was aghast to see that their main witness was my man. But I felt like less of an idiot, since he had managed to convince a whole team of ITV researchers.