Have you ever been in an argument (debate or otherwise) and the person with whom you are arguing ignores your point and responds, "Oh yeah? Well what about you?!?!?" If so, you have experienced my least favorite logical fallacy: Tu Quoque, or 'You too.' This is a common argument tactic often developed on the playground sometime around first grade. I also think of it as 'Justifying your actions through the actions of others.'
The problem with this fallacy is that it avoids the argument all together - usually in the face of a good point. Often people that employ this tactic have no reasonable response so they side-step and try to turn the argument onto the other person. Gee...does this sound like any politician you may have heard?