Today I thought I broke the numpad on my keyboard; none of then numbers were working. Switching on and off the Num Lock key didn't work. However, anytime I hit a number and held the key down, the mouse cursor would move across the screen. By some sort of black magic, I found a secret keyboard combo to turn on Mouse control via keypad.
Solution to turn it off
System ⇾ Preferences ⇾ Assistive Technologies ⇾ Keyboard Accessibility ⇾ Mouse Keys
Uncheck 'Pointer can be controlled using the keypad'
I must admit, however, this can be a handy feature when working with a mouse-less machine.