Existing usability error classification methods focus on the magnitude of errors. There’s a need to categorize not only the magnitude but also the root cause of such errors. In this presentation we propose a method to categorize errors as either “Mistakes” (cognitive-based errors) or “Slips” (accidental errors in space/time). For example, a user may incorrectly type their PIN as 1-2-3-4 instead of the correct 1-2-3-5 due to misremembering the number (a mistake). Alternatively, a user may intend to type the correct PIN, but accidentally selected the last digit as 4 instead of 5 (a slip). Categorizing errors this way helps researchers provide directed design recommendations. Specifically, the design might be improved by helping users remember their code to prevent a mistake or having input keys with larger touch targets/better spacing to prevent a slip. Furthermore, we provide specific examples regarding how this approach has aided in gaining buy-in from stakeholders.