If you log into the Google AdSense Site Diagnostics screen and see that Google AdSense is not serving ads on specific pages due to Robots.txt files and you begin to freak out a bit thinking that Google Adsense ads are not being served, like I just did, don’t worry as there seems to be a bug in the system and Jen Slegg is on the case.
From Jen’s post at Search Engine Land:
I spoke with Google, and this error is being fixed. However, it does not affect ad serving, if someone views your site in Google cache, they will still see ads which are credited to the publisher with their intact publisher IDs still in the code. It just makes it hard to find any other issues in the site diagnostics tab, especially if you have a site with a lot of cached page views.
Thanks for looking out for us all Jen