So some guy answered and I realized I had dialed the wrong number. I apologized and hung up. Unfortunately I was left with a dilemma. Was the number on the piece of paper in my hand the incorrect number, or had I accidentally mis-dialed one of the digits. The call was over and I had no way of knowing what number I had actually dialed, so I could not check the piece of paper against the number I dialed. If I dialed the number on the paper again then I might end up talking to the same guy, which would be embarrassing, and it is a bit rude to disturb the same stranger twice.
If I had been using a phone with a call-log, the screen might show me the last dialed number and I could check, but most simple land-line phones lack such a feature.
Bear this dilemma in mind when designing screens that tell the user that they have done something wrong. If you are telling someone ‘Invalid ZIP code’ or ‘Too many digits in account number’, make sure that the data that your interface is complaining about is still visible. Too often, especially on the small UI available to many embedded systems, the error message hides the data that the user just input, leaving the user wondering what exactly they did enter. The user may know what they intended to enter, but after the fact, they may be unsure. This often leads to the user entering exactly the same data a second time, because they assumed that they made some kind of a slip the first time.
Once you reach the point where you tell the user that they input the wrong data, it is often nicer to have the option of correcting the previously entered data rather than starting over. If I got the ZIP code wrong, I may just want to fix one digit, rather than starting all over. For a string as short as a ZIP code, starting over is a minor inconvenience. As the data they enter gets larger the price of restarting the data entry gets higher. So try to find a way to save the user’s work so they can reuse the majority of it.
Handing user errors is a challenging area. Read more about it in an old article of mine in Embedded Systems Programming magazine called To Err is Human