Sometimes when returning my laptop from sleep (which locks/unlocks the screen) I have a stale unlock dialog on the screen. I can't do anything with it. It's painted on top of everything, but doesn't react to any clicks (they go through, IIRC). I can't get rid of it unless I restart kwin. The problem is that it's pretty much in the way. Not sure if it also happens when only locking the screen (i.e. without actually sleeping), at least I could not trigger it that way up to now. This is also not specific to the unlock dialog, I also saw this happening with the plasma notification popups, in the past. Compositing is enabled. Reproducible: Sometimes The machine has a Nvidia Geforce 320M with binary driver, currently 304.64. Compositing is enabled.
*** This bug has been marked as a duplicate of bug 306186 ***