XKB has a feature referred to as "MouseKeys" or in the case of xkeyboard-config "PointerKeys". Once enabled, the keypad controls the pointer movement and can be used to emulate button events. The default keymap hat the PointerKeys toggle on Shift+NumLock.
Like so many other things, it is a low level feature that has virtually zero desktop integration. The only way to find out whether it is on or not is to hit a key and see the mouse cursor move. That of course requires you to already know about the feature.
If you've accidentally enabled it the only thing you may notice is that your keypad stopped working. Worse, with a nasty bug we had in servers 1.7 and later (fixed now), the mouse buttons would get stuck when PointerKeys was used intermixed with physical mouse button presses. I've received a fair number of bug reports and other complaints about this feature that I finally decided to remove it from the default options. Sergey has pushed the patch to the xkeyboard-config repository over the weekend, so expect that in xkeyboard-config 2.1 and later, PointerKeys will need to be explicitly enabled.
You can do so with the XKB option "keypad:pointerkeys", or by ticking the "Toggle PointerKeys with Shift + NumLock" checkbox in your desktop keyboard configuration UI.
Like the changed zapping behaviour and the middle button emulation default (both of which have completely failed to bring the world to an end, despite claims to the contrary), this is only a changed default. The feature is still there and can be re-enabled on-the-fly or permanently if needed.