For example, this user said he saw an ad on his site. He then went to the AdSense preview tool and the ad was not there.
Here is how the AdSense publisher described the problem:
I've noticed this for months now, but, recently especially when I try to track down the URL of MFA spam sites (in order to comp-filter them) which put different URLs in the ad than they actually redirect to, that they don't even show up in the Adsense preview! When you input the showing URL the domain is non-existant. They show up on my page, but when I click on the preview tool they are nowhere to be found, thus it is harder to filter them as you have to risk accidently clicking them to get the google-code and parse it.
I did confirm this, but there seems to be a couple, "me too" posts in the thread.
Forum discussion at WebmasterWorld.