The document discusses 5 common mistakes made when choosing or managing a security system: 1) Ignoring the security rating of the building, 2) Lacking clear security objectives and strategy, 3) Being confused by new technologies, 4) Using wrong products and unreliable suppliers, and 5) Underestimating the importance of maintenance. It recommends evaluating the current security level, creating a security plan with objectives and roadmap, educating oneself on technologies without needing expert knowledge, choosing reliable suppliers with good service records, and implementing a maintenance plan to keep the system reliable over time.