Here's an extract from Principles of Interactive Computer Graphics, Newman and Sproull, 2nd edition. This book came out in 1979 yet its observations are just as pertinent today, twenty years later. And the frightening thing is that it might still be pertinent twenty years from now, too.

No single component of an interactive program is more unpredictable in performance than the user interface, i.e. the part of the program that determines how the user and the computer communicate. It is unfortunate that this should be so, for user-interface design has a particularly strong impact on program acceptability as a whole. Our inability to predict user interface performance makes it particularly likely that users will react in unexpected ways when they first use the program. The biggest surprises often occur when the programmer sits down with his first user to explain the operation of the program:

Programmer: Now that you've drawn part of the circuit, you might want to change it in some way.

User: Yes, let's delete a component. How do we do that?

P: Point at the menu item labeled CD.

U: CD?

P: It stands for 'component delete.'

U: Ah. Well, here goes...hey, what happened?

P: You're in analysis mode: you must have selected AM instead of CD.

U: Funny, I was pointing at CD. How can I get out of analysis mode?

P: Just type control-Q.

U: [types C-O-N-T-R...]

P: No, hold down the control key and hit Q.

U: Sorry, silly of me...OK, I'll try for CD again.

P: Maybe aim a bit above the letters to avoid getting into analysis mode---no, not that much above---that's better.

U: Got it!

P: Now point to the component to delete it.

U: OK...nothing's happening; what am I doing wrong?

P: You're not doing anything wrong; you've deleted the component, but the program hasn't removed it from the screen yet.

U: When will it be removed?

P: When you type control-J to redraw the picture.

U: I'll try it...there we are; but only part of the component was removed!

P: Sorry, I forgot: you have to delete each half of the component separately. Just point to CD again.

U: Very well...now what's happened?

P: You're in analysis mode again: type control-Q.

U: Control...where's that Q? There is it...hey, why is the screen blank all of a sudden?

P: You typed Q, not control-Q, so the program quit to the operating system. I'm really sorry, but we've lost everything and we'll have to start all over again.