This time I crashed Opera and now I can reproduce it with a simple script so I submitted a bug report. I’ve tried it on several Windows XP machines running Opera 9.02 or 9.00 and all of them crashed. In case you have some other Opera version or the latest version running on other OS you can try if this bug affects your browser (and let me or Opera know). To do this click here, press “Open popup” button and “Insert stuff” button in the popup. Keep in mind that your Opera will most likely crash so don’t do this in a browser with important tabs open.
As far as I understand the source of the problem is that I create image object in one document and try to insert it in the other document while closing the document in which that object was created.
Update: newly released Opera 9.1.0 still crashes.