Update
Kev (below) has identified that this is caused by a conflict with firefox. Sounds logical. Switching to version 1.2.0b4 or disabling firefox reportedly fixes the problem.

The Issue:
When browsing a site with one of Doubleclick‘s ads on it, sometimes the ad will take over the whole page (likely due to malformed HTML in the iframe or some type of javascript error), effectively rendering the page useless. The back button doesn’t work because it just keeps taking over the page when you press ‘Back’. I usually end up just closing the tab and forgetting about it. That being said, I imagine other annoyed users are doing the same. This behavior is occurring across multiple very large sites (list below). It makes me wonder if these publishers are aware of potential dollars they are wasting by compromising the user experience in such a big way.

Over the past month, I have taken screenshots of this issue across various sites. It could be a conflict with a FireFox plugin (although I only use Web Developer Plugin and Firebug), but that’s unlikely. This issue seems to be caused by firebug. This issue has been recorded in firefox 2 and 3. By looking at the status bar text which reads something like ‘Transferring data from m0.2mdn.net’ or ‘Read m1.2mdn.net’, and by a quick glance at the source of each of these broken pages, I can quickly conclude that the problem lies somewhere within Doubleclick’s ad code.

yelp, YouTube, Gizmodo, and LinkedIn have all been captured with this problem.

I also wonder if Doubleclick is aware of this issue. They deal with very large advertisers and I can’t imagine something like this going unnoticed for too long.

Has anyone else seen this bug while browsing popular web sites?