Sometimes it happens when it redirected to dedicated page of ad. I just use the last page to appropriate page I wanted to look. I have some offending ad redirect listed at my webpage privacy policy blocked:

adnxs.com
vindcosuite.com

I'll check the blocked page on my other computer as well.

Other times when the ad is out of location, usually below the ad banner box. I'll monitor which offending ad domain names that causes the problem.