The vulnerability, rated "highly critical" by Secunia, can be exploited by malicious people to take complete control a user's system.
From Secunia's advisory:
The vulnerability is caused due to an error when processing HTTP responses having a malformed "Content-Length" header. This can be exploited to cause a heap-based buffer overflow via an overly large 64-bit "Content-Length" value, having the higher 32-bit part negative.
The vulnerability is confirmed in version 10.50 for Windows. Other versions may also be affected.
In the absence of a patch, Opera users are urged to avoid browsing to untrusted Web sites or switch to an alternative browser.