All the chewy goodness of fallacious reasoning in new fun-sized bites!
According to Dr. John Nolt et al. “Tu quoque (‘you too’) arguments attempt to refute a claim by attacking its proponent on the grounds that he or she is a hypocrite, upholds a double standard of conduct, or is selective and therefore inconsistent in enforcing a principle.” Tu quoque is a specific form of ad hominem that, in other words, seeks to discredit an argument by discrediting its proponent.
My Christian neighbor believes that we should not sin.
My Christian neighbor sins all the time.
Therefore, we should sin.
Since it does not follow that the hypocrisy (or inconsistent conduct) of a proponent renders his argument invalid, this argument commits a tu quoque fallacy. For more on this, see “Do the Actions of Christians Determine the Truth of Christ?”
The best way to point out someone’s fallacy is by using the Columbo tactic as laid out in the Funsized Tactics series. There are plenty of excellent resources available in book, e-book, or PDF format. Two good places to start, with regard to formal and informal fallacies, are A Preface to Philosophy by Mark B. Woodhouse and Schaum’s Outlines: Logic by John Nolt, Dennis Rohatyn, and Achille Varzi.
 John Nolt, Dennis Rohatyn, and Achille Varzi, Schaum’s Outlines: Logic, 2nd ed. (New York: The McGraw-Hill Companies, 2011), 197.