In March of last year, Google came out with their own form of display ad retargeting, they named it remarketing.
Remarking basically lets you show ads on third party web sites to people who have visited your web site. If they visited your web site to learn about flowers you sell, they can then see your ad on a flower blog that runs AdSense. That, in a nutshell is remarketing or retargeting.
I spotted a WebmasterWorld thread where one advertiser is convinced that his pixels on his commerce site is being used not just for remarketing purposes for his site but also for his competitors.
We have overwhelming evidence that suggests Google is utilizing the clickstream data collected from our client's Adwords retargeting pixels (i.e. "Audience") to serve competing ads on the Adsense network.
I do not follow Google's display network that closely but I've never seen a complaint like this before.
I assume if it works this way, I am sure it can also work in the reverse where your competitor's data helps you as well. Is that a bad thing? Seems so?
Forum discussion at WebmasterWorld.