Try loading in SWFs created with Blaze (the artist tool) into your flex app, and you're in for an even more huge world of hurt.  You can fume memory (crash the browser within 30 seconds) without even writing a single line of code, just by having an artist placing certain kinds of keyframed animations and fonts.  We spent weeks trying to explain this problem to the absolutely idiotic Adobe Support Gold, and after months we got something like "seems like something might be up.  we'll have our engineers look into it.  Can you send us another test case?"