Why are you passing a [notify] in your $_GET in the first place?
No idea. Nothing I have actually coded myself.

I've done some searching and the only references I can find to notify and get would be in the shopping cart class, for example:

zen_redirect(zen_href_link($_GET['main_page'], zen_get_all_get_params(array('action', 'notify', 'main_page'))));

but this is a stock file and I don't really understand what I'm looking at or for.

Can you suggest more that I can add to the debug info to help locate this? Or a construct I could search for in the code?