If your site runs Doubleclick Flash banner ads in iframes, you may have a problem. It seems some of the Flash ads set the browser focus to themselves. This means that, as the ads run in an iframe, the focus is taken away from the page your visitor is expecting to be on.
There are two major implications. Firstly any visitor not using a mouse may well have difficulty getting focus back to the main page in order that they can read and navigate through it, not least because they won’t expect the page focus to have changed without them knowing. A clear accessibility failing.
Secondly any user will have unexpected results when printing the page - all they will get is the ad printed out - and they will probably blame that on your site.
The problem was pointed out by Jude Robinson who is currently doing an excellent job accessifying Nature.com. See this page for a fine example of the work and a nefarious Doubleclick ad.