https://bugs.adobe.com/jira/browse/FP-1770

if you have a reference to Keyboard or Capabilities (or, probably, a number of other native Classes as well) in an event handler, it will loiter as a pair of Class and Object references when the enclosing swf is unloaded via Loader.unload().

for example:
private function onKeyDown (evt:KeyboardEvent) :void {
    if (evt.keyCode == Keyboard.SPACE) { doStuff(); }
}

must be replaced with:
private function onKeyDown (evt:KeyboardEvent) :void {
    if (evt.keyCode == 32) { doStuff(); }
}

what a pain.