The following error can even be found the FancyApps page itself. Go to http://fancyapps.com/fancybox/ in Internet Explorer 9 for example. Go to Internet Options > Advanced > Disable script debugging (Internet Explorer). Click on the YouTube (iframe)-link and close the movie. You'll see the following error appear onscreen:
Line: 1 Error: '_flash_removeCallback' is undefined
Any idea on how this error can be avoided? Thanks in advance!