Ad hoc updates to system catalogs are not allowed.

Error Message:
Msg 259, Level 16, State 1, Line 1
Ad hoc updates to system catalogs are not allowed.

Severity level:
16.

Description:
This error message appears when you try to run an UPDATE statement against SQL Server's system tables.

Consequences:
The T-SQL statement can be parsed, but causes the error at runtime.

Resolution:
Errors of the Severity Level 16 are generated by the user and are corrigible by the user. The statement cannot be executed this way. You should refrain from manipulating system tables as you might render the database unusable.

Versions:
All versions of SQL Server.

Example(s):
UPDATE sysobjects SET name = NULL

Remarks:
In SQL Server 2000 there was an extended option that would allow running modification queries against system tables. In SQL Server 2005 this method has been removed. Although there are still other ways, all are unsupported and you're better off calling the SQL Server Product Support.