The X Window System and the curse of NumLock

Ordinary modifiers are normally straightforward, in that they are additional keys that are held down as you type the main key. Control, Shift, and Alt all work this way (by default). However, some modifiers are sticky', where you tap their key once to turn them on and then tap their key again to turn them off. The obvious example of this is Caps Lock (unless you turn its effects off, remapping its physical key to be, say, another Ctrl key). Another example, one that many X users have historically wound up quietly cursing, is NumLock. Why people wind up cursing NumLock, and why I have a program to control its state, is because of how X programs (such as window managers) often do their key and mouse button bindings.

Chris Siebenmann

I always have an applet in my KDE panel that shows me if I have any sticky modifiers enabled without realising it. On some of my keyboards, this isn't always easily noticable, especially when you're focused on what's happening on your display. A little icon that only shows up when a sticky modifier is engaged solves this problem, as it immediately stands out in your peripheral vision.