Recently, I've noticed that my MacBook Pro 5,1 (mid 2009) running Snow Leopard 10.6.6 doesn't respond when I press CTRL+3 or CTRL+4.
When, for instance, I'm in an application that has no bindings for the CTRL-keys and I press CTRL+2, I will hear the OS X alert sound, because the application receives a key combination it doesn't understand. This is the case for all CTRL+n combinations, except 3 and 4.
No matter where I try it — even in places where pressing the aforementioned key combination would have an actual effect — nothing happens.
But it doesn't just seem to ignore the keypresses; it numbs the keyboard. If I, for instance, hold CTRL and press 5 and 6, I will hear two alerts. One for each unknown key combination. If I then do the same, but with the numbers 4 and 5 (which usually are responsive), there's no sound. In fact, no matter what key I press when CTRL is being held down after having pressed 3 or 4, nothing happens.
At first I figured I probably added some shortcut to the key combinations by accident, but I can't find it anywhere. In System Preferences → Keyboard, I've restored everything to default, but that didn't seem to change anything.
I've tried with both the onboard keyboard, a wireless Apple bluetooth keyboard and a cabled USB Apple keyboard.