Explanation:

This is somewhat of a diversionary tactic. By pointing out that someone else did a similar wrong, it is the hope of the propagandist that the receiver will say, "Oh, well I guess it's okay then." Of course it's not okay.

Examples:

(i) NFL football players justifying the use of a "slashing" gesture across their throat by saying that professional wrestling is something children also watch, and it's a lot worse.

(ii) President Clinton and his administration argued that there was nothing wrong with having sex with his intern Monica Lewinsky because other public officials had done similar things. (It turns out they had not, but even if the charge was true, it is irrelevant.)

(iii) Johnny stole candy from the store and explains that it wasn't wrong because Jimmy did it too.

Proof:

Show that accusing one of a similar wrong is completely irrelevant to the situation at hand.