Somehow, somewhere along the line my number pad stopped working as a number pad and instead started controlling the mouse cursor (in Windows it is referred to as mousekeys).
I use my number pad on a very regular basis and soon became extremely frustrated at this.
Here’s the solution:
System – Preferences – Assistive Technologies
Clear the checkbox [ ] Pointer can be controlled using the keypad
No need to reboot, it is instantaneous and sticky across reboots.