Recently Google has been sending me emails about 404 errors while crawling my Joomla site.
The URLs are not the primary URLs but seems to be a secondary setup. So if the main site is(for example): www.mysite.com/index.php/interestingpage The error comes from www.mysite.com/~alternativename/index.php/interestingpage.
~alternativename is a alternative product name we have used. I don't recall setting it up but the 1st level works. However as the crawler goes deeper into the site, the alternative path starts failing.
I have not changed my site recently but inn the last few days, I am getting 100s of errors from Google crawling.