g1smd - 4:03 pm on Jun 27, 2011 (gmt 0)
I agree this is mainly a problem when relative links are followed and misinterpreted. Combined with mod_rewrite rules that don't validate the leading folders in the requested URLs, you can quickly have a crawling nightmare on your hands.
Make sure the site uses linking that BEGINs with a leading slash and make sure your rewrite rules are tightly coded. Avoid ambiguous patterns.