Help! What are Yahoo doing?

[internetheaven]

Yahoo is listing several URLs that just don't exist and have never existed on my server:

www.example.com/Related.htm
www.example.com/sis.htm
www.example.com/kumiawase.htm
www.example.com/lfobp.htm
www.example.com/envhome/lunarpages.htm
www.example.com/mayor/tesco_co_uk.htm
www.example.com/FBIMemberShip.htm
www.example.com/cracked.htm

Obviously the last two are the most worrying. I can't find any sites linking to those URLs so have absolutely no idea how Yahoo got them. But as I use a 404 error code in my htaccess file they are all showing with the same content as my main page which means that there are several copies of the same content and my site is no-where to be seen in the results.

No other search engine (Google, MSN Beta etc.) has these results, it is only Yahoo.