The are broken custom 404 responses out there which use relative paths, so if, for example, this URL is requested:
They include something like:
As part of the standard menu.
However, when converted to absolute, that URL will become http://stuff.com/non/existent/path/sitemap and cause yet another similar 404 response leading to an infinite loop.
The Spider needs to be updated in order to identify custom 404 pages and keep track of their relative links.
My idea wouldn't guarantee that the resulting path would be a 404, just that it's likely to.
So, gotta close this, I don't like messing with coverage based on heuristics.