SQL Server 2005 ‘Login failed’ (error 18456 state 16)

The event log of one of our SQL Servers filled up with events 18456 (every minute).

Failure Audit: Login failed for user ‘\’. [CLIENT:]

Looking in the SQL Server log, I found some extra information.

2011-10-18 12:52:00.55 Logon       Error: 18456, Severity: 14, State: 16.

I found this MSDN blog post, explaining most of the State codes, except for state 16.

ERROR STATE

ERROR DESCRIPTION

2 and 5

Invalid userid

6

Attempt to use a Windows login name with SQL Authentication

7

Login disabled and password mismatch

8

Password mismatch

9

Invalid password

11 and 12

Valid login but server access failure

13

SQL Server service paused

18

Change password required

Digging a little further, I found that State 16 usually means that the target database cannot be located. I most cases, this is due to an SQL Server Job that targets a deleted of offline database.
Deleting or disabling the job gets rid of the errors in the event log.

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