My mom’s telephone comes with a speaker that allows you to make calls without using the receiver. Of course, there are two buttons to control the speaker. One to decrease and one to increase the volume. So far so good.
After a power outage, the phone decided to reset the volume for its ringtone. It was barely audible, so my mom went to fix that and pressed the volume-up button she found on the keypad. This was, however, the aforementioned control
for the speaker — not the ringer.
To make adjusting the speaker volume even more comfortable, pressing the controls turns on the speaker giving direct audible feedback. Nice. It just happens to also pick up the line to provide any audible signal.
So instead of adjusting the ringtone’s volume, my mom picked up the phone line with the speaker which she usually never, if ever, does. I even suspect her not knowing about the speaker function and all the things that come with it.
There is another button. The ringtone-configuration-button. Conveniently marked with a bell. Not too bad. When you press it you get into the ringtone-configuration-mode. You notice because the phone starts ringing (my mom thought I called her, but that is another story). What you don’t notice is that some of the buttons now have a new meaning. The number keys select different ringtones and the speaker volume controls now adjust the ringtone volume.
This is not really confusing for a reasonably technical person, but it should be obvious to anyone. My mom operates her Mac Mini with ease but that phone is stuck with some metaphors that are just confusing when they are approached intuitively.
Don Norman in his The Design of Everyday Things talks about various incarnations of this — and how to avoid it — in detail. If you have to have modes, map them to an intuitive mental model, not some artificial or even technical one. And make absolutely clear in which mode you are and what that means for the controls that might have changed their meaning.