In this paper, we describe a lightweight solution for protecting real-time collaborative editing systems against Byzantine faults. We observe that a centralized coordination algorithm not only reduces the complexity of the editing system, it makes easier to harden the system with Byzantine fault tolerance. We perform a comprehensive analysis of the potential threats towards collaborative editing systems and introduce a set of Byzantine fault tolerance mechanisms without requiring any additional redundant resources. If the system has sufficient redundancy, such mechanisms can be used to ensure strong protection against various malicious faults. Even without sufficient redundancy in the system, our mechanisms would still help limit the damages caused by a faulty user.