I'm getting the dreaded

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error message on the Shipping section of the One Page Checkout. The only shipper is FedEx. Is there known science for debugging these sorts of issues without doing a deep dive on how Magento renders the Shipping Step, and how that kicks off API requests for rates?

In other words — what tools exist in the UI for debugging these issues, and if not tools exists what are the best classes to add temporary debugging tools to?