source: http://www.securityfocus.com/bid/9203/info

It has been discovered that the Mozilla browser is prone to a URI obfuscation weakness that may hide the true contents of a link. The problem occurs when a user@location URI is formatted in such a way that a NULL byte is located after the user value. It is said that, when doing a mouseover of such a URI, it will cause it to only display the contents of the user value, not the entire link.

This could be used in conjunction with other URI obfuscation attacks and browser vulnerabilities to trick a user into following a malicious link. 

http://www.trusted.com%00@www.malicious.com