False error links in Google sitemap.

What are the sources of them?

[Vadim]

Hello,
In the HTTP error sections of the Google sitemap, Google reports that the link

****/default.htm
Gives 404 not found
("default.htm" is the real name)

Since I know exactly that there has never been a page with the name "default.htm" in my site, where could Google take this link from?

Or more general question. Does Google always takes the error URLs from the same site or it uses the inbound links form the other sites?

Vadim.