funny that you mention this. yesterday i noticed on my sites blank spaces in the ad blocks showing from time to time (oddly colored, slightly brighter than my selected background color). was about to post something here if others had the same issues. however, problem seems to be fixed by now. although i suspected that this wasn't necessarily a problem but a test: show no ads in an ad block if the stats say that it's better for overall performance.. but doesn't make sense, does it?