.shtml page acting as a virtual directory?

Found this in my logs...

[wruppert]

Someone added extra stuff to one of my few .shtml pages as in:

http://www.example.com/ssi-page.shtml/extra-stuff

Instead of returning a 404, Apache returned ssi-page.shtml, sort of. The CSS and images were missing (relative links), so it was unstyled html.

What is happening? Is there some rewrite trick to avoid this?