Seaquel
SQL Server error code

SQL Server error 666

SQL Server raises error 666 at severity 16, which marks a problem with the query or data that you can correct.

The maximum system-generated unique value for a duplicate group was exceeded for index with partition ID %I64d. Dropping and re-creating the index may resolve this; otherwise, use another clustering key.
Msg666
Severity16
Group0–999

Source: Database Engine events and errors