Some pop internet sources classify the tu quoque argument as a fallacy, but that's erroneous:
I am using ad hominem in the way Peter Geach uses it on pp. 26-27 of his Reason and Argument (Basil Blackwell 1976):This Latin term indicates that these are arguments addressed to a particular man -- in fact, the other fellow you are disputing with. You start from something he believes as a premise, and infer a conclusion he won't admit to be true. If you have not been cheating in your reasoning, you will have shown that your opponent's present body of beliefs is inconsistent and it's up to him to modify it somewhere.As Geach points out, there is nothing fallacious about such an argumentative procedure. If A succeeds in showing B that his doxastic system harbors a contradiction, then not everything that B believes can be true.