I can't find a better article, but heres the gist:

Two labs tested a B-sample from Iban Mayo after the Spanish Cycling Federation appealed the results from the French Lab that in the past has leaked positive drug tests from Landis, Rasmussen, and was also criticized by Armstrong. Neither of the other labs could get a positive on Mayo. Sadly, this type of confirmation was not done for either Landis or Rasmussen, and the corrupt lab retested the B sample and always got a positive. I always felt Floyd got screwed, and this is why.