Classification of errors: Slips vs Mistakes
Whenever we try to learn a new skill, be it skiing, typing, cooking, we are bound to make mistakes. In most situations, it is not a bad thing because the feedback from making errors can help us to learn. When learning to use a computer system, however, learners are often frightened of making errors because, as well as making them feel dull, they think it can result in catastrophe.
Studies have extensively analyzed human errors and distinguish between two main categories based on the user mental model: Mistakes, and slips.
The first category mistakes, that happens when you do what you have to do but you have a wrong mental model of what you ought to do. An incorrect action is taken based on an incorrect decision. For example, trying to throw the icon of the hard disk into the wastebasket, in the desktop metaphor, as a way of removing all existing files from the disk is a mistake. A menu option to erase the disk is the appropriate action.
Mistakes are a part of being human. In a way or another, they will happen to anyone but designers need always to keep in mind to provide better feedback or make clear to the users what the options are. Jakob Nielsen would say "Help users avoid conditions where errors arise or warn users before they take risky actions".
The second category slips, that happens when you accidentally do the wrong thing but you have the right mental model of how the system works. Slips are unintentional. They happen by accident, such as making typos by pressing the wrong key or selecting the wrong menu item by overshooting.
The most frequent errors are slips, they can further be classified into six types: capture errors, description errors, data-driven errors, associative-activation errors, loss-of-activation errors, and mode errors.
A capture error is a frequently done activity that takes over (captures) the intended action.
A description error is when the correct action is carried out on the wrong object.
A data-driven error is when unconscious processing of external data interferes with what you had intended to do.
An associative-activation error is when internal thoughts interfere with what you are supposed to be doing.
A loss-of-activation error is when you forget to do something in the mid-flow of an activity
A mode error is when you think you are in one state but in fact, you are in another
Again, slips will happen but to prevent, or limit, them, designers can try to improve the ergonomics or visual design of the user interface - spreading things out, making icons bigger, texts easier to read, and using colors wisely. Systems providing warning and alerting messages can play an important role in helping users deal with their errors. System error messages should be designed to be informative and honest.