UsabilityDesignPatterns/HiddenMode

Even worse than a regular UsabilityDesignPatterns/ModeError Mode because the state of the mode is not visible on screen, and in some cases the user may not be aware that a mode even exists - only that their interface is behaving strangely and they don't know why. Hidden modes are one of the worst things you can do to users.

One way to do this is by having check boxes buried in the preferences (see UsabilityDesignPatterns/PreferenceDialogFromHell) that completely change the behavior of the rest of the interface. If one of these gets changed somehow, it can be very hard to find it and set it back.