You receive an "Error 605" error message when you run a query with the optimizer hint NOLOCK or you set the transaction isolation level to READ UNCOMMITTED in SQL Server (235880)

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Typically, access to data that is being changed by either another user or process is denied because of locks put on the data. However, the NOLOCK and READ UNCOMMITTED commands enable a query to read data that is locked by another user. This is referred to as a dirty read because you can read values that have not yet been committed and are subject to change.

When a query that is using NOLOCK or READ UNCOMMITTED tries to read data that is being moved or changed by another user, a 605 error occurs. The severity level of 605 errors occurring during a dirty read operation is 12 versus a severity level of 21 when reading committed data. If a 605 error that has a severity level of 12 occurs, it is most likely a transient 605 error and does not indicate a database consistency problem. To verify that it is a transient 605 error, rerun the query later.

If the error persists, remove the NOLOCK hint or set the transaction isolation level to READ COMMITTED and verify that a 605 severity level 21 does not occur. A 605 level 21 error indicates possible database corruption. If a 605 level 21 error occurs, see to SQL Server Books Online for more information and contact your primary support provider for more help.