Change is a primary source of bugs in software development, a realization that comes from years of experience rather than textbook knowledge. Early in his career, the author believed that bugs primarily stemmed from coding errors and logic mistakes. However, he learned that many issues arise after systems have been tested and deployed, often triggered by unexpected changes. The author highlights various factors that contribute to this phenomenon, such as changes in dependencies, which can introduce subtle incompatibilities and regressions. Distributed systems add complexity with their constant fluctuations, where assumptions about stability often lead to failures. Configuration issues also play a significant role in introducing bugs, particularly when environments differ or settings are altered without proper documentation. The author emphasizes that many software engineering practices focus on managing change. Techniques like immutable data structures, pure functions, and modularization are all strategies to limit the impact of change. Tools such as version control and containerization further support this goal by helping track and control changes. Beyond technical factors, change can also come from organizational shifts and evolving business needs, which can complicate systems and introduce uncertainty. The author urges engineers to identify the sources of change within their systems—whether from dependencies, infrastructure, or configurations—to better anticipate and manage their effects. Recognizing that change isn’t inherently negative but can drive progress, he stresses the importance of understanding its dynamics to prevent unexpected surprises.