CVE 2005-0593
Firefox before 1.0.1 and Mozilla before 1.7.6 allows remote attackers to spoof the SSL "secure site" lock icon via (1) a web site that does not finish loading, which shows the lock of the previous site, (2) a non-HTTP server that uses SSL, which causes the lock to be displayed when the SSL handshake is completed, or (3) a URL that generates an HTTP 204 error, which updates the icon and location information but does not change the display of the original site.
See the 
CVE page on Mitre.org
for more details.
