I ran into this curious predicament earlier today when I found that the numeric keypad was not working in Kubuntu. Now, I have a KVM setup here and therefore could easily verify that the keyboard itself was working fine. This meant that the problem was a software issue.
Oddly enough, after some testing I realised that some of the keys triggered the context menu of the mouse rather than their intended functions (or simply playing dead). This caused my synapses to flash into a flurry of "aha!" moments as I remembered that I had activated the "Mousekeys" feature during a demonstration for a friend a few weeks back. Once I remembered this, the solution was easy:
Toggling the Mousekeys feature can be done using the keyboard short-cut CTRL + SHIFT + Num Lock. This feature can also be toggled and otherwise configured via its settings page within Kubuntu's System Settings. This page is sequestered within the Hardware -> Input Devices -> Mouse sub-menu and its Mouse Navigation tab.
Hope this helps somebody out there :)