random // pages shown as 404 errors

In the error pages list, there are many pages which are valid URL's,
except "//" appears between the domain name and path, instead of the
usual "/". From a link: search, there are no links anywhere to these
URL's, let Google Sitemaps shows 'em as 404 errors.

If there are no such links in my site (both Xenu and link: on G,Y,M
agree), where do these come from?