If you want to install Zend Server CE on Debian Linux (e.g. Ubuntu Server) with apt-get and follow one of the many installation guides out there you might stumble upon this error, like I did.
http://repos.zend.com/deb/zend.key 404 Not Found
The solution is simple: The URL of the key has changed recently! Use http://repos.zend.com/zend.key and it will be fine!
Of course, the official guide has it right.