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:
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When I change the workspace theme in KDE System Preferences to something else than "Air" and back again, the problem disappears.
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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.
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I created a new clean user account and logged in with KDE – the same problem appeared – so it's not that I have some outdated files in my .kde4 config directory.
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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.
Best Answer
I've had this exact problem on all my newer KDE installations. It seems to be a bug in KDE though I've not seen anything specific on it. A quick fix that I've used is to change the height of the toolbar. It seems that if you change the height to something smaller, the problem disappears forever. It's just that the default height is buggy.