Not too long ago the snack machine at my company was replaced by a new model. No big deal. It still had the same basic assortment of chips, candy bars, and peanut butter crackers. But, there was one big difference: the numbering for the items had changed.
In the old machine, everything had a two-digit number kept in simple sequence: 23 for Doritos, 24 for the Snicker's bar next door. The new machine had a different number scheme, and a three-digit one to boot. Why on earth start numbering the snack cells all 100? What a waste of finger power.
Ah . . . but not so fast. With the old machines, every now and again I would look to buy the crackers in slot 19, see they were priced at 55 cents, and type 55 on the keypad (transposing the price for the slot number). And instead, item #55--Pringles, yuck!--would come spinning out of the machine.
Not possible with the new machine. No prices began with one, so if you even hit the 1 key first you got an error from the little screen.
Ingenious! And good user interface design, too, because it anticipates the user's mistakes and, rather than saying in the classic socially-maladjusted programmer way, "too bad, you screwed up, live with the Pringles", it says, "Hey, buddy--are you sure about that?" and allows you to correct the error of your ways.
And that's good interface design.
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