Rick, the OPC debug file is generated (and accumulated) on a customer-id basis so that I can infer the transitions from the main (checkout_one) to confirmation (checkout_one_confirmation) pages.
Note that the error message is indicating that something has changed between the main-page's submission and the confirmation page's processing and "normally" isn't displayed.
Send me a copy of the myDEBUG and OPC debug files and I'll see what's up.
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