404 errors occur when a website has been moved or a page on the site has been changed. Also, technically speaking, the actual server is reachable, but the page is not.
For example, if you have moved your about page from example.co.uk/about-us to example.co.uk/about then this page link has now changed. So if you visit the older link there won’t be a page on for the server to find.
Companies often use 404 pages to promote a sense of humour, as some use funny graphics.
Click here for more information on how to fix 404 errors and what they mean for your Google Rankings.