As you can see, I've been reporting bugs in Opera since 2003. Any bug I can easily make a test case for, I've stored here; so just the rendering bugs pretty much. I've reported several more, but they're mostly to do with the Opera interface itself.
A number of years back, Opera abandoned it's own in-house rendering engine (nicknamed "Presto") and went with the Chromium engine instead. Thus, all of the bugs that still remain "uncrossed-off" below, are almost certainly no longer relevant. C'est la vie.
opacity
transitions cannot be cancelled
(August 17, 2012)
border="0"
(October 25, 2010)
position:relative;
(February 15, 2009)
select
elements always returns 0
(October 23, 2008)
select
elements fail when mouse drag leaves the element
(October 8, 2008)
scrollTop
property of a hidden textarea
fails
(September 23, 2008)
onmousedown
property of document set to return false;
disables all focus
(May 30, 2008)
height
do not collapse table cells
(November 14, 2007)
page-break-inside:avoid;
is ignored on tables
(November 8, 2006)
selectedIndex
(August 29, 2006)
data:
URIs + background
attribute
(November 6, 2005)
tfoot
misrendered
(September 15, 2005)
overflow:hidden;
& position:absolute;
(March 23, 2005)
overflow:auto;
truncation
(March 17, 2005)