According to reports at DigitalPoint forums, the latest beta release of Google Toolbar hijacks 404 pages as shown in the image above.
It’s not clear from the reports as to whether this occurs only when no customized 404 page is available on a specific site, or with every 404 page. I also can’t test the theory, least the only beta version of Google Toolbar I could find was for Internet Explorer.
A little research by the writer would have shown this "feature" was unveiled in a Google post in December - and it only affects non-customized 404 pages. A big bruhaha over nothing?