Offensive cyber operations (OCOs) have become a prominent tool in the arsenals of state and non-state actors, offering capabilities ranging from espionage to destructive attacks. However, the interconnected nature of cyberspace introduces a complex challenge: the potential for unintended consequences, commonly referred to as collateral damage. This systematic literature review examines the current understanding of collateral damage stemming from OCOs. We synthesize definitions, analyze the technical and legal challenges associated with predicting and mitigating such effects, and explore the implications for international law, particularly the principles of distinction and proportionality. Our findings reveal a persistent gap between the theoretical frameworks and the practical realities of preventing unintended harm in a highly interdependent digital environment. We highlight critical areas for future research, including improved methodologies for effects assessment, enhanced legal interpretability for cyberspace, and the development of robust strategies to minimize spillover.