Microsoft Knowledge Base Article 914032 contains the following introduction:
In the Active Directory directory service, operations master role connectivity problems are not reported in Event Viewer. No processes run on the original release version of Windows Server 2003 that monitor the health or availability of operations master roles. Therefore, it is frequently not clear that an operations master is missing until an operation that depends on the operations master is performed.
Operations master roles may be silently removed when a domain controller suffers a hardware or software failure and the domain controller is not restored from backup. You can also silently remove an operations master role from the domain or from the forest by running the following command:
Windows Server 2003 Service Pack 1 (SP1) records when an operations master role is removed. Certain conditions that affect operations masters cause Windows Server 2003 SP1-based domain controllers to log a Warning event in the Directory Services event log.