There are reports that the new beta version of the Google Toolbar is acting a bit evil. Reports are that when you install the toolbar and surf to a 404 page, a page not found, Google will replace the 404 page with their own page.
I have not tested this myself, because I am on a Mac and this is an issue with a Windows toolbar. I do not know if it affects all 404 pages, both default 404 pages and custom 404 pages or just default 404 pages. In any event, this is what Google uses to replace your 404 page:
I find it humorous that Google, who has horrible 404 pages, is replacing 404 pages for other sites with the toolbar. Besides for the humor and irony in this, replacing a 404 page with something else is purely evil in many webmaster's minds.