Phantom page in Google search results

For my site, but not a page on my site

[androidtech]

Hello,

I did a "site:mysite" google search recently to see what pages were indexed. Much to my surprise, a page called "register.asp" showed up as a supplemental search result. When I clicked on Google's "cached" link, I saw my custom error page which is named "error404.html", NOT "register.asp".

How can this be? I don't run any ASP pages on my site at all which is hosted on a Linux server; only HTML and PHP.

Should I be concerned about this?

Thanks.