My personal guess: A queue calls into an external 3rd party service, but the external service is not transactional, so it can receive and store the request, but

My personal guess: A queue calls into an external 3rd party service, but the external service is not transactional, so it can receive and store the request, but before it finishes executing, it throws an exception and returns an error code. The queue keeps retrying into the external service. While the external service keeps printing cards.