Five worst Opera bugs in my opinion
1. Terrible box-shadow redrawing (DSK-29255, DSK-281717)
Seriously, this is something that Opera developers really didn't make good. Not even sufficient. The only case when everything looks nice is when there is some element with with box-shadow on static page without scrollbar and :hover selector. There are some examples:
2. Wrong rendering of SVG background when zoom is other then 100 (DSK-285664)
Test page: http://www.operacesky.net/bugs/files/00050a.html - try to zoom there ... there shouldn't be any "spaces" between the rectangles
3. Wrong content of "dragged transparent copy of image" after scrolling (DSK-298900)
There isn't probably much people that care about this one but I hate it. Go on any site (where you can scroll) with image(s). And try to drag some of the images. There should appear the transparent copy of the image below the cursor - that's ok. Now scroll and try ti again. What happens? There isn't the image in the transparent copy but some other part of the site.
4. Position fixed is not fully fixed
If there is some fixed element on the web page and someone scrolls with mouse wheel the element is "jumping"
5. Buttons in Flash WebCam/Mic confirmation box are not working (DSK-302717)
You can test it here: http://midomi.com
So this is in my opinion the five worst bugs in Opera. Now it's time for you to create your own list here in comments or post it to your own blog =o).