There is a general need to understand software defects and the ability to detect defects during different activities. This is particularly important in relation to software process improvement, where one objective may be to decrease the number of defects. The Personal Software Process (PSP) has gained attention during the last couple of years as a way to individual improvement in software development. Thus, the PSP is an interesting starting point to understand software defects and in particular the detection process. The paper presents a study of software defect detection for 59 students taking a PSP course. In summary, the study provides valuable insight into software defect detection in the PSP. Some of the results are interesting not only for the PSP, but from a general perspective in the understanding of software defect detection. The understanding of software defects forms the basis for improving the reliability of software.