A friend had a simple question:

If I use the mx:Image tag to load an image and get a 404, how do I handle that?

I jokingly said that the tag should support an onLoad type event so you could check the http status codes. Turn out I wasn't far from the truth. Mike Kollen (The Uber Flex Instructor) sent me this simple block of code as an example of how to handle it:

private function ioError():void { movieImage.source = "assets/no_image.gif"; }

Talk about simple. Half of my battles with Flex are simply learning how to do stuff - and luckily when I do learn - it typically turns out to be easy to implement.