Some people are running into problems adding SWF content to Facebook Profile pages. There seems to be a caching issue with some versions of Internet Explorer.

This issue showed up for me when developing a Facebook application that has a Flex component in the user's profile. I found that clearing my IE cache solved the problem for my purposes.

If you have an application where other user's have added your app, obviously you can't ask everyone to clear their cache. In that case check out the Facebook Developers Wiki for more information on the issue as well as a nice workaround.