When describing the NodeId attribute, Part 3, Section 5.2.2 says:

A Server shall persist the NodeId of a Node, that is, it shall not generate new NodeIds when rebooting.

However in Part 5, Section 6.3.1 when describing the NamespaceArray property, it says:

NamespaceArray defines an array of namespace URIs. This Variable is also referred as namespace table. The indexes into the namespace table are referred to as NamespaceIndexes. NamespaceIndexes are used in NodeIds in OPC UA Services, rather than the longer namespace URI. Index 0 is reserved for the OPC UA namespace, and index 1 is reserved for the local Server. Clients may read the entire namespace table or they may read individual entries in the namespace table. The Server shall not modify or delete entries of the namespace table while any client has an open session to the Server, because clients may cache the namespace table. A Server may add entries to the namespace table even if clients are connected to the Server. It is recommended that Servers not change the indexes of the namespace table but only add entries, because the client may cache NodeIds using the indexes.

Nevertheless, it might not always be possible for Servers to avoid changing indexes in the namespace table. Clients that cache NamespaceIndexes of NodeIds should always check when starting a session to verify that the cached NamespaceIndexes have not changed.

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These two sections describe mutually exclusive behavior for the server. I think Part 3 should change to say "should" instead of "shall", and perhaps add a note about how it should be avoided if possible.