I bought an F350 back in 2010. It was a used '09 and I bought it at one of the big local Ford dealerships.
21 months later, I was getting ready to sell it. Also looking at possibly trading it in, a dealership ran a Carfax. One week before the dealer ran that Carfax, they posted the truck had been wrecked in the front end before I bought it.
In other words, Carfax posted in May, 2011 that the truck had been wrecked in August 2009! I had no idea. Carfax only tells you what is reported when its reported. But if something isn't reported and it takes awhile, it means nothing.