Suppose that someone criticizes you for telling a white lie. If
your critic is a notorious liar, you would probably be
tempted to say, "Look who's talking!" This response is certainly
understandable--no one enjoys being censured by a moral inferior--but it is fallacious.
It's a species of ad hominem known as the tu quoque ("you're
another") argument. The fact that someone else is guilty of an
accusation doesn't prove that you are innocent.
A related version of the ad hominem fallacy occurs when we
attack someone's position by claiming that it is inconsistent
with his or her practice or other positions.