Opera Browser 7.23, and other versions before 7.50, updates the address bar as soon as the user clicks a link, which allows remote attackers to redirect to other sites via the onUnload attribute.
https://exchange.xforce.ibmcloud.com/vulnerabilities/16131
https://euvd.enisa.europa.eu/vulnerability/EUVD-2004-2252
http://www.securityfocus.com/bid/10337