From: Sean Wilson on
> Basic Debugging: Add a break point to the beginning of your preparemovie
> and/or startmovie handlers and step through the program.
>
> If the break point is not hit, then the handler is not being run. If it is
> hit, then step through each line until you hit the line with the error.

Unfortunately, like the OP said, the error only occurs when the movie is
published, and one doesn't have access to the debugger at run-time. A
useful alternative is to pepper handlers with 'put' commands (and enable
a run-time message window) or 'alert's to narrow down where errors are
being thrown