The only way for a machine to pass the (hypothetically optimal) Turing test would be if the machine fully "understood" human consciousness. Since we are unable to see that ourselves, I'm just hoping we don't accidentally create something we can't understand that understands us. At which point, there is no way to determine a successful passing of the Turing test! And further, we as a species would have failed the reverse Turing test for sure! Matt Groening should have written an episode of Futurama about this.