When the screen is unlocked the lock dialog is sometimes left present on top of the desktop. The desktop is accessible but the lock window cannot be interacted with (i.e. it doesn't respond to mouse clicks and cannot be selected with window selection through Alt+Tab). The dialog cannot be removed. Locking and unlocking again doesn't help; the lock window is still visible afterwards.

I've attached a screenshot which was taken by selecting to screenshot only a window under the cursor and clicking on the lock dialog. As you can see, the underlying browser window has been shot instead while the lock dialog still being visible in front of it.

Reproducible: Sometimes

Steps to Reproduce:
1. Lock the screen.
2. In my case the computer was idle for the night so the display was turned off. This might not be necessary to reproduce the problem.
3. Unlock the screen by typing the password.

Actual Results:  
The desktop appears but the lock dialog is still visible and cannot be removed.

Expected Results:  
The lock dialog should not be shown after unlocking.

I'm using Kubuntu 12.04 with KDE 4.9.4 and AMD Catalyst drivers on MacBook Pro. I have an external monitor attached to the laptop.