>The solution is to use RETURN DODEFAULT() in the bottom-level form.unload() method.

Exactly so.
Another common problem comes from forgetting to pass (optional) parameters to a parent in DODEFAULT().
Like keypress - DODEFAULT() doesn't pass the keypress information and the parent class code doesn't work right. Had me scratching my head - then hitting it on the desk!

Now I have a mousepad with "bang head here" printed on it.
Doesn't hurt so much :)