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I've noticed it myself a couple of times and on pages that have had good targeted ads for years. One minute the ads aren't there - return to the page a few minutes later and they're back.
Someone else started a thread about a large difference in AdSense impressions recorded and the number of site visitors.
It got me to wondering if this is happening more often than is being noticed? I have no idea if someone visiting my site(s) right now is being shown ads.
Has anyone else found blank spaces where ads should be? I don't know a good way to check for it other than random checks, that's how I found the problem on my sites.
I don't know a good way to check for it
One needs to exclude extremities (sites or channels whose actual page and ad impressions are low), those fluctuate heavily and will never give you any solid data to base observations upon.
Second ALWAYS configure alternate ads to your channels and monitor daily from your own site statistics how many times the alternate ad file was served.
Finally all you need to do is look at a long term daily AdSense reported ads served vs. Site reported page impressions served graph.
If you are in tune with your site stats, you will know within few hours if something is out of whack even without the use of graphs.