Most important, most means most, feature that a browser must have is that it should crash when something went wrong.
This is the reason why I stop using firefox. And now, seems like Opera has the same problem. Even worse, I can say.
So, my advice is to set Opera to crash when is unavailable (it can't be used, it's not responding).
After appending some html code with javascript, an wrong function appendet the same code over and over (loop forefer). Making this, the browser stopped working and my processor was up to 80-90% used by Opera. My computer was freezed for a while... until I closed the browser using task manager.
I hope you won't say that this is my problem cause I'm developer and i'm a bad programmer...
PS: Windows 7, 64 - Opera 49