Four Causes of Errors
When we observe errors in usability tests we find it helpful to identify their causes and find they generally fall into four broad categories.
Slips: You can't eliminate all those "fat finger" errors or typos but seeing a lot of slips can be a good indication to reduce required fields or data entry where possible.
Mistakes : When we see users entering the wrong format in a field it's usually a good indication that some field-hint, an auto format or some code that gracefully strips non-numeric characters might reduce these mistakes.
User Interface Problems: Errors caused by the interface are the ones we're most interested in as we can usually do something about these. If users continue to click on a heading that's not clickable (mistake) or look for a product in the wrong part of the navigation then there's probably something about the design that we can improve.
Scenario Errors: No matter how sophisticated and realistic our usability tests are, there is some degree of artificiality to them. For example, if you want to test how well users can pay a credit card bill online then you have to provide them with fake data and a test system. Inevitably we see errors related to the artificial scenario as users see balances and transactions that are foreign to them. We can't do much about these errors except note that they are unlikely to be encountered in actual use.