I am developing kde game, korners.
Now a problem appeared which I didn't have later.
kDebug() says:

korners(8126) KornersRender::loadTheme: discarding cache
korners(8126): Failed to lock file "" , last result = 2
korners(8126)/kdeui (KIconLoader): Unable to lock pixmap cache when trying to discard it

application works as ever but I think because of this startup time increased. It has a pause when discarding cashe.
Here are the sources:
http://korners.googlecode.com/files/korners.0.83.tar.gz

KPixmapCache has been heavily reworked for KDE 4.5.

As a workaround, remove the contents of /var/tmp/kdecache-$USER/kpc/

As I discovered, probably cache wasn't initialized. To initialize it I called isEnabled() and as I see it forced initialization, and this worked the problem around. People on kde dev list say it's a bug.