I've just stumbled across a re-draw bug in Opera's handling of opacity which isn't present in the current stable version (9.24). The bug is present in both Windows and Mac versions of the browser.
I've put together a sample page and set an opacity of 0.7 on the first two paragraphs. On first load they're rendered correctly but if you scroll down the page so that they disappear out of view and then back up again to bring them back into view you'll see that the backgrounds haven't been re-drawn correctly. For elements below the fold (i.e. outside the default viewport) with opacity set their backgrounds are never drawn. I tried several different values for opacity and all exhibit the bug.
I contacted the good folks at Opera with details so hopefully they'll be able to include a fix before the stable version goes out.
This bug seems to have been fixed in the latest build so I assume it should make the release version.
Thanks to Chris Mills from Opera for checking it out and getting back to me.
Have been using Opera a fair bit lately and can't say that I have noticed any issues with this.