One of my users complained about a 300x250 leaderboard that was sitting on top of my content. Basically this is not possible, because I do extensive tests in different browsers.
So I asked him to send me a screenshot and indeed, there was one ad sitting in the correct place and another leaderboard right below it, hiding some of my content.
It turned out that a plugin in Chrome and Firefox was replacing my ads on his computer showing other ad networks, but did it in a wrong way. I would have never known if there were not two ads displaying at the same time due to a bug of this crappy plugin. After deleting the plugins, everything went to normal.
This really annoys me! I found my peace with people using adblocker, but I consider this as stealing.
Besides that, it might trigger some false alarm at the Googleplex giving me more troubles.
Has anyone experience with it? Is there a way to prevent that?