That is forbidden, but does not invalidate. A judgment of invalidity is serious business and should not be lightly undertaken by a layman -- on whom the burden of proof rests.
I believe that if the very act of transubstantiation would cause desecration to the Eucharist, no transubstantiation occurs. Since an ignoble material inherently desecrates the Eucharist...
But you do make a correct point. I should state myself more carefully: Based on Pyro's description, there is grave concern that such a mass is invalid, and hence it is prudent not to receive communion
If an ordained priest said the required words over the host/wine, then it's a valid Eucharist. Method of distribution does not reverse-transubstantiate the body and blood of Christ back to wheat. If it was not wheat, but rice (or some other concoction), then it would be invalid.