Active Directory replication failed because of a schema mismatch between the servers
The replication operation failed because of a schema mismatch between the servers involved.
All domain controllers in a forest must hold a consistent view of the schema partition. When the schema definition on the source and destination DCs diverges — most commonly after a schema update has been applied to some but not yet all DCs, or during DC promotion when the new DC's schema is out of sync — replication of attribute changes that reference the newer schema definitions will fail with error 8418.
The replication operation failed because of a schema mismatch between the servers involved.
Why it happens
A schema update (e.g., adprep /forestprep, an Exchange schema extension, or Lync/Skype schema prep) was applied to the Schema Master and has not yet fully replicated to all domain controllers in the forest.
An underlying replication failure (such as a DNS, network, or access denied error) is preventing the schema partition from replicating to one or more DCs, leaving them with an older schema version.
A new domain controller is being promoted using Dcpromo and the helper DC it contacts for schema replication has not itself received the latest schema update.
The source domain controller's schema partition contains duplicate multi-valued attributes or corrupted attribute/page data in the directory database.
A manual or automated change was made directly to the schema on one DC without allowing it to replicate before subsequent changes were attempted.
How to fix it
Check current schema version across all DCs: `repadmin /showattr * "cn=schema,cn=configuration,dc=domain,dc=com" /atts:objectVersion`. All DCs should report the same value.
Identify which DCs have the most recent schema and which are lagging: `repadmin /replsummary`. Focus on DCs showing 8418 errors as the source or destination.
Check for underlying replication failures that are preventing schema partition replication: `repadmin /showrepl`. Resolve any errors (DNS, RPC, access denied) on the schema partition before investigating the 8418 further.
If the schema mismatch is transient following a recent schema update, allow time for the schema to propagate. Schema updates replicate like any other change; if replication is working, the error should resolve itself within one replication cycle.
If the mismatch persists, force replication of the schema partition from the Schema Master to all DCs: `repadmin /syncall /AePdq` run from the Schema Master.
If the error occurs during DC promotion, ensure the helper DC being used for promotion has the latest schema version before re-attempting Dcpromo.
If database corruption on the source DC is suspected (duplicate multi-valued attributes or corrupt pages), engage Microsoft Support. Export schema data using `ldifde -f schema_source.ldf -d cn=schema,cn=configuration,dc=domain,dc=com -s <SourceDC>` from both the source and destination DCs to compare schema definitions and identify the divergent attribute.
