Iframes are not necessarily evil, but the don't always behave the way you might expect.
This page tests one particular iframe issue that appears in WebKit and Gecko: Iframes that are added to a document and then "moved" are re-loaded following the move operation. Publishers that implement this technique to defer loading of ads until the end of a page are then double-counting impressions when those tags deliver to firefox.
The ad below implements an AdKeeper button. This button code makes an impression tracking call a few seconds after the iframe has loaded. The impression tracking call is served from the impl.ackak.com domain and can be seen in Firebug's Net tab.
Click the button to move the frame above the content. Note that the iframe is re-requested. Then note how, a few seconds after the frame lodads, the request for impl.ackak.com is made again. This indicates Firefox has re-loaded the frame and re-executed the javascript. This impression has now been counted twice even though the ad is only displayed once.