12.15 changelog: http://www.opera.com/docs/changelogs/unified/1215/
-- Fixed an issue where the search bar's default engine could be overridden by third-party apps.
-- Fixed a moderately severe issue, as reported by Attila Suszter; details will be disclosed at a later date.
-- Fixed: RC4 encryption protocol is vulnerable to certain brute force attacks. Due to the time this amount of requests takes, this is not a practical attack against most users.
-- Fixed: Cookies can be set for a top-level domain. In some cases, this may confuse a site's cookie handling, causing it to mistake that cookie for one of its own, and reusing it for authentication without modification. This could lead to the user's accounts being compromised on that site.
12.16 changelog: http://www.opera.com/docs/changelogs/unified/1216/
-- Opera Software recently experienced an attack on its internal infrastructure. Following best practices, Opera is replacing signing certificates in Opera with newly issued certificates. (The time window for possibly-compromised 12.15 version downloads was very narrow in time, but in principle some downloads could have been performed therein with flawed/faked certs, so Opera bumped the version to assure new clean certs would be deployed to users).