This happens when transparency is used without compositing.
Created attachment 147347 [details] Patch
Comment on attachment 147347 [details] Patch Whoops!
Comment on attachment 147347 [details] Patch Needless to say, tests would also be great - but I don't know of a good way to do this for WVI :/
Comment on attachment 147347 [details] Patch Clearing flags on attachment: 147347 Committed r120254: <http://trac.webkit.org/changeset/120254>
All reviewed patches have been landed. Closing bug.