Little research has been done examining the role of errors in learning computer software. It is argued, though, that understanding the errors that people make while learning new software is important to improving instruction. The purpose of the current study was to (a) develop a meaningful and practical system for classifying computer software errors, (b) determine the relative effect of specific error types on learning, and (c) examine the impact of computer ability on error behaviour. Thirty-six adults (18 males, 18 females), representing three computer ability levels (beginner, intermediate, and advanced), volunteered to think out loud while they learned the rudimentary steps (moving the cursor, using a menu, entering data) required to use a spreadsheet software package. Classifying errors according to six basic categories (action, orientation, knowledge processing, seeking information, state, and style) proved to be useful. Errors related to knowledge processing, seeking information, and actions were observed most frequently, however, state, style, and orientation errors had the largest immediate negative impact on learning. A more detailed analysis revealed that subjects were most vulnerable when observing, trying to remember, and building mental models. The effect of errors was partially related to computer ability, however beginner, intermediate and advanced users were remarkably similar with respect to the prevalence of errors.