A study has been made of the software errors committed during development of an interactive special-purpose editor system. This product, developed for commercial production use, has been followed during nine months of coding, unit testing, function testing, and system testing. Detected problems and their fixes have been described by testers and debuggers. A new fault categorization scheme was developed from these descriptions and used to classify the 173 faults that resulted from the project's errors. For each error, we asked the programmers to select its most likely cause, report the stages of the software development cycle in which the error was committed and the problem first noticed, and the circumstances of the problem's detection and isolation, including time required, techniques tried, and successful techniques. The results collected in this study are compared to results from earlier studies, and similarities and differences are noted.