When a swf is embedded in a page and is not cached locally in the browser, two requests for the swf are fired by the browser. Here is a simple example.
The first request returns an expected 200 OK, and the second request triggers a 304 Not Modified after the first request has completed, indicating the file is now cached locally.
Why is this 2nd request fired and how can I prevent it?
I believe this can also be triggered by running Firebug - try checking your browser requests using LiveHTTPHeaders (or some other http sniffing app like Charles) while you have Firebug disabled.