Are there valid instances of the fallacy of affirmation of the consequent?

according to massey there is no theory whatsoever behind the standard treatment of fallacies. nevertheless, he agrees that the so called formal fallacies can falsify his claim. since formal fallacies are invalid patterns of argumentation proscribed by logical theory, he purports to show that they can, anyhow, yield valid arguments. massey chooses the fallacy of "affirmation of the consequent" and provides one example of it to support such claim. his underlying point is that while proofs of argument validity can be considered definitive and taken to have theoretical legitimacy, proofs of argument invalidity cannot. in this paper i will challenge massey's example of an argument that instantiates the pattern known as "affirmation of the consequent" and yet is valid. i will argue that his example is not a genuine case of affirmation of the consequent, but a mere sham argument on which he has performed a trick.