Why does human action trigger unintended effects? This question has intrigued social scientists for decades – and only now, with the help of network science, have we began to unravel the forces that set those chain reactions in motion. The unintended consequences of human action take many forms and are captured by many metaphors: from self-fulfilling prophecies to cumulative effects; from negative feedback loops to virtuous circles. Communication networks pulsate at the heart of all these processes of change. Digital technologies, with their trove of data and tools, allow us to tap into those networks, and solve the puzzle of why our actions generate outcomes that we did not intend or envision. This talk will offer an account of the progress made in recent decades, and dive into the details of my own research, which aims to illuminate how networks facilitate large-scale coordination -- and dynamics of change that lay beyond the control of any one individual involved.