I can trigger this bug on my system (Sony VGN-NR laptop) by pressing non-functioning Fn key combos, and S1 button. Pressing a Fn combo that is working, Fn+F4 (volume up) for example, does not trigger the bug.
Logging out and in from the system (not just out of KDE) makes everything go back to normal, as in krunner starts when Alt+F2 is pressed. If I just restart KDE instead, krunner will also start after pressing Alt+F2 but only for a short span of time (maybe 3 seconds?), after which xfrun4 will "replace" krunner.
In System Settings - Autostart, I have the following two entries (they are disabled, though):
xfce4-settings-helper-autostart
xfconf-migration-4.6