I am generally impressed by the St Pete Times Politifact.com fact checking site. Like many fans, I was surprised when they classified the DCCC ad on the Ryan budget resolution as pants on fire false.
As noted by Jonathan Chait, this can't be explained by actual falsehoods in the ad, since there is arguably one (is Medicare eliminated if it is replaced with a very different program called "Medicare").
My theory is that it is a matter of subliminal suggestion. The ad closes with an elderly man who says "... fire ..." and then is shown dancing with no pants on.
The sequence suggests "pants on fire" doesn't it ? I have contacted Politifact asking them if this is the cause of the rating. However, I promised not to tell if they reply that it is, so you'll just have to guess.