There seems to be a major mouse handling bug in the tab bar in konqueror. I'm not sure if it's KWin related or not, but it seems to always trigger when I accidentally start dragging a tab. Restarting kwin gets me 1 mouse click before the mouse becomes unusable again. Basically when I start dragging a tab, it gets into drag mode but never drops properly. I'm not sure this is reproduceable every time, but it has happened 3 times in the past 4 days now. The mouse seems to be grabbed in drag mode and never exits. So far the only way to cleanly get control back seems to be to log out and log back in. This also seems to be a recent development in HEAD.
I can not reproduce it in konqueror 4.1. Is it still present for you?
Please reopen if you suffer this bug again.