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.”
Google has switched to more user-friendly error messages that 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.
Related: How to Track 404 Errors
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)