When unmapping a key code in the capplet (or changing existing mapping), the key
that is unmapped gets cleared its keysym without ever restoring the original value.

Try this (pick a sound/multimedia binding like Next track):
1. Assign a key "w" (you can use any modifier).
2. Remove the assignment via Backspace.
3. Now you cannot use the "w" key anywhere in X - it is dead, because it got its
keysym removed.

Only logout/login restores correct behavior.

The bug is in gnome-settings-daemon/gnome-settings-multimedia-keys.c:290
(update_kbd_cb). We unhook the key in unhookup_keysym, but then the key is clear
and can no longer be used!

The correct solution would probably be:
a) to remember the original value after login and rebind that value

or

b) when doing the keyboard update, instead of unhooking individual keys, reset
to the session default keyboard layout.