You're seeing this because the page that you tried to access does not exist. This might have happened because of a problem with a link within the site, an out of date bookmark or because you clicked on an out of date link from another site.
If you used a link within the site, you could hit the back button and try that link again. If you were outside the site then your best bet is to go to the Slicethepie homepage and navigate from there.
If you have no joy and are convinced the page should exist, please let us know.