At one day (it has been for a while now) the background of KDE main toolbar started looking really strange: screenshot (it looks to me as the alpha channel of the background image is missing).
Debugging I've done:
When I change the workspace theme in KDE System Preferences to something else than "Air" and back again, the problem disappears.
When I choose any some other theme than the default "Air" there is no problem, except that I don't like these other themes as much.
I created a new clean user account and logged in with KDE - the same problem appeared - so it's not that I have some outdated files in my .kde4 config directory.
I have tried reinstalling several components of KDE that could effect this, but without success - might have not been the right ones, haven't done a complete reinstall of KDE.
It probably happened after some update of my Gentoo system, but I may not remember correctly, as I have looked for a solution to this for quite some time now.
If only I knew were to look for to find this particular image file and see if there is something wrong with it... but I've grepped and located and finded in /usr/share quite a bit and I don't seem to understand where are those KDE workspace theme files located.
Using KDE 4.4.5 with the default "Air" theme.