Certain page address (URL) constructs can cause Opera to allocate the wrong amount of memory for storing the address. When it then attempts to store the address, it will overwrite unrelated memory with attacker-controlled data. This can lead to a crash, which may also execute that data as code.
Opera Software has released Opera 12.01 and Opera 11.64, where this issue has been fixed.
Thanks to Andrey Stroganov for reporting this issue to Opera Software.