Need help troubleshooting Webmaster Tools 404's

Somehow Google Webmasters is interpreting links to other sites and appending my url in front of them, causing 404 errors.

For example:

It shows this url as not existing on my site: http://www.mustangevolution.com/parts/part_details.asp?PartKeyField=5937

and then says that the page that linked to it is here: http://www.mustangevolution.com/forum/f142/t28046/

However, a link doesn't exist on that page... BUT this link does: http://www.fordracingparts.com/parts/part_details.asp?PartKeyField=5937

How is it taking that external link and then replacing the fordracingparts.com with mustangevolution.com?

There are more examples: http://www.mustangevolution.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7932168486&rd=1&sspage name=STRK%3AMEWA%3AIT

Obviously that is a ebay url but somehow it has made it my url...

Google does this all the time. I hate to say it's normal, but it is.

they don't exist, so they are in fact 404's and thus show up there. How/why google tries to index these urls is a mystery.