If I follow the instructions of the third tutorial and create a ReceivedFormSubmission.ss template in mysite/templates/, containing only $OnCompleteMessage, when I execute the form the server goes nuts (error 500 in a remote one, halt on local WAMPServer).
If I delete ReceivedFormSubmission.ss and I restart the server, the form is correctly executed, the mail is sent and the OnCompleteMessage appears in the browser.
If I create a ReceivedFormSubmission.ss containing something as <p>Test</p>, the form is correctly executed, the mail is sent and 'Test' appears in the browser.
Tested with three different projects/databases.
I hope it's something that happens only in special cases or beginners will get mad! BTW, this tutorial should be rewritten, since it does not explain many UserDefinedForm features. On the other hand, I understand that these forms are being moved to a different module.
Sorry about my English.