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That's no longer a problem because it appears all ads in the content network are now disabled when you display pages with AdSense via Google cache links.
Now the fact that cached pages used to display off topic ads was silly because Google knew where the pages came from so there was no rhyme or reason that AdSense would go off topic whatsoever unless there was an internal disconnect between Google Search and AdSense. This made no sense anyway because Google claims both Googlebot and AdSense Mediabot share the same cache of the pages, so it never made sense because both the webmasters and Google were losing money from this ridiculous situation.
Now, the pages with AdSense display nothing whatsoever so it's a complete loss for all involved.
No PSA, no off target ads, just a big blank spot making us all look stupid.
There is one caveat to this in that I've seen a couple of premium publisher sites still displaying AdSense ads in the cache, but none of the 30+ publisher sites I tested for regular AdSense ads showed anything at all in Google cache.
Personally, I'd prefer the ads enabled when cached pages are viewed and the same ads properly displayed that would be seen on the same page when in context of the website, so everyone involved could make some money, but that's no longer an option.
What's the reason for this current change?
Could it be a simple technical malfunction?
Is it possible that AdSense in cache was disabled so that the cache pages are truly "fair use" and Google isn't monetizing them?
Could it be a sign of the times [webmasterworld.com] and with some belt tightening that Google doesn't want to share revenue when the visitors are on their site? Are they trying to get 100% of the PPC revenue when the visitor clicks BACK out of the cache?
No clue, purely tossing ideas around.
I'm not sure of the reason, but it's a huge change without as much as a peep about it from anyone anywhere!
It may be related to allowed sites feature. If it is enabled ad will only display on selected domain and it will not display on other places.
Oh and ads are showing just fine on Google cache pages (though a little off-topic), so I suppose it makes sense there hasn't been a peep about it from anyone anywhere. Can't think of a reason why they're not showing for you though. Did you provide the other people on IM with a direct link to the Google cache, or did they go there on their own?
It's the data center that makes the difference.
If you get cache from here, no AdSense ads display!
If you get cache from here, you get AdSense ads!
So cut & paste your cache query to either of the links above and you should see the different behavior depending on the cache server.
No clue why, but at least we know where it happens now.
Thanks to purplecape for the clue in helping figure this out ;)