A million years ago, I helped developed a web service that was invoked via a custom action for a SharePoint Designer workflow.  This week, the client wanted to move it to production (finally!) so we did.

The custom action worked fine, but the web service it invoked did not, giving us this error:

System.InvalidOperationException: This operation can be performed only on a computer that is joined to a server farm by users who have permissions in SQL Server to read from the configuration database. To connect this server to the server farm, use the SharePoint Products and Technologies Configuration Wizard, located on the Start menu in Administrative Tools.
   at Microsoft.SharePoint.Administration.SPWebApplication.Lookup(Uri requestUri)

Turns out that I forgot to add the service to the SharePoint application pool in IIS.  Once I did that, it worked fine.

This MSDN forum posting gave me the clue I needed: http://social.msdn.microsoft.com/Forums/en-US/sharepointdevelopment/thread/2c97c004-7118-4e06-a62c-b2b0ac07ac99

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