How to debug a runtime stack underflow error?

I'm really struggling to resolve a stack underflow that I'm getting. The traceback I get at runtime is:

VerifyError: Error #1024: Stack underflow occurred.

at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at flash.net::URLLoader/onComplete()

This is particularly difficult to debug because when I run in debug mode it does not happen at all. It only happens when compiled as a release.

Does anyone have any tips on how to debug a Stack Underflow? Are have a clean explanation of what that means for Flash?

In case it helps, this error is occurring when I click a button whose handler makes an RPC call, which uses a URLLoader, an AsyncToken, and then invokes the set of AsyncResponder instances associated with the AsyncToken. With some server-side logging as well as some logging hacked into the swf, I know that the UrlLoader is successfully doing and GET'ing a crossdomain.xml file, is correctly processing it (ie: if I wreck it, I get a security error), and is also successfully completing the "load" request (the server sends the data). The underflow seems to be happening in the Event.COMPLETE listening/handling process (as is, of course, implied by the traceback as well).

mxmlc used = from flex_sdk_4.5.0.20967

Example player (I've tried a few) = 10.2.153.1

UPDATE: My specific problem is solved... but I'm leaving the question as-is since I would like to know how to generally debug such a problem, rather than just getting my specific solution.

In my code I had the following Application definition:

<s:Application height="100%" width="100%"
                              xmlns:fx="http://ns.adobe.com/mxml/2009"
               xmlns:s="library://ns.adobe.com/flex/spark"
               xmlns:mx="library://ns.adobe.com/flex/mx"
               initialize="InitData();">

Note