Google intercepts 404 pages

We tried to give a heads-up in a couple places. The Toolbar beta 5 announcement on the Google blog mentioned “You’ll get suggestions instead of error pages: If you mistype a URL or a page is down, now the Toolbar will give you that familiar “Did you mean” with alternatives, like when you do a Google search.” And the John Mueller did an excellent run-down for webmasters when he talked about the Google toolbar beta on Google’s official webmaster blog. Here’s the part of John’s post that probably interests you:

404 errors with default error pages
When a visitor tries to reach your content with an invalid URL and your server returns a short, default error message (less than 512 bytes), the Toolbar will suggest an alternate URL to the visitor. If this is a general problem in your website, you will see these URLs also listed in the crawl errors section of your Webmaster Tools account.

If you choose to set up a custom error page, make sure it returns result code 404. The content of the 404 page can help your visitors to understand that they tried to reach a missing page and provides suggestions regarding how to find the content they were looking for. When a site displays a custom error page the Toolbar will no longer provide suggestions for that site. You can check the behavior of the Toolbar by visiting an invalid URL on your site with the Google Toolbar installed...

Source: www.mattcutts.com