VUPEN security reported a vulnerability “ Content-Length - Header Buffer Overflow”.

A vulnerability has been identified in Opera, which could be exploited by remote attackers to compromise a vulnerable system. This issue is caused by a buffer overflow error when processing malformed HTTP "Content-Length:" headers, which could be exploited by remote attackers to crash an affected browser or execute arbitrary code by tricking a user into visiting a web page hosted on a malicious web server.

Read more here

Opera officials says that if your system is DEP (Data Execution Prevention ) there is nothing to worry.  If you are using Windows Xp with SP2 read more on How to enable DEP here

But the Register reports there are methods which can bypass DEP - Read more here.

So we recommend you that stop using opera 10.50 until opera fixes this bug.  you may use previous version of opera.