The Twitter fail whale made it famous but someone hitting your 404 page has long been an early indicator of a potential major problem with a site. You can do a useful hack with chartbeat alerts to make sure you are instantly aware of anyone arriving at your 404 page instead of their intended destination.
First check that your 404 page has the same global header and footer as your other pages (so that the chartbeat code is on the page). Then go to ‘Add An Alert’ and select:
Send an email when total visitors on a specific page (type in the address of your 404 page) goes above 0 people.
Then choose if you wish to receive the alert by email or SMS and press ‘Add Alert’.
That’s it! Now you’ll always be instantly aware of anyone hitting your 404 page.