If you happen to type a web address on Google that doesn’t exist (e.g., google.com/yahoo, google.com/adsence), you are no longer shown the default 404 Error Page that says “The requested URL was not found on this server.”
Since errors like 404 are like Greek to most people, Google is taking a more user friendly approach and they now try to guess the page you are looking for.
For instance, if you type google.com/email, it suggests you the Gmail website instead of displaying the regular “Page Not Found” error.
Similarly, a typo like google.com/blog will throw in suggestions pointing to the Google Blog Search Engine. A very neat approach.
Update: You will still see the same old 404 page on Google if your erroneous URL has a file extension (like google.com/labnol or google.com/secret.php)
Related: How to Track 404 Errors