After locking the screen, when you move the mouse to show the login prompt, the desktop is briefly flashed on screen, bypassing the screensaver/lock before the dialogue window comes up.
Sometimes I experience the same bug, today screen wasn't even blanked - I moved the mouse, screen turned on and I've seen my desktop with unlock dialog over it. "Blank screen" screensaver, KDE 4.9.5, gentoo, XRender compositing with "disable for fullscreen windows".
This shouldn't be a problem with the new screenlocker in 4.10 *** This bug has been marked as a duplicate of bug 291078 ***