Definition:

An error is any action that causes an unwanted result. Errors fall into two types: slips and mistakes. A slip is an action that was not what was intended. An example is immediately clicking the OK button on a popup asking to delete your entire hard drive. You didn't *want* to delete everything but you did unconsciously. Mistakes are errors that occur because a user's intention is not correct but that action for that intention is sound. A common medical mistake is to treat the wrong symtoms with a specific medicine. Administering a prescription for stomach pains is the right way to treat a patient for a food allergy, but not if the patient's appendix is about to burst.

Interaction Design:

The two most common ways to account for slips and mistakes in interaction design is to warn a user if you can tell that something has been done wrong or offer the user a way to undo their last action if it was unintended. Common examples of this are forms that check your credit card information before allowing you to order a product online. This situation also falls under the 80/20 rule because most of your customers are going to use your online checkout process and some of them will encounter an error at least once. If these errors are well designed they process can fall under the aesthetic-usability rule as well.

User Experience Design:

Automobiles have many chances for errors to occur every second that they are in use and the experience of driving is heavily influenced by preventing these errors. Most people take for granted the interlock between your gear shift and the ability to take the key out of your ignition. Imagine you're parking on a hill and commit a slip -- you forget to put the car in park. Instead of your car rolling into the one behind it, the interlock tries to prevent you from doing this by holding your key. This is effective because the feedback is immediate -- you've done something wrong. In combination with lack of consistency -- this doesn't usually happen, what did I do differently -- allows the slip to be caught and corrected quickly.