Tu quoque

Tu quoque is a discussion technique that intends to discredit the opponent's argument by attacking the opponent's own personal behavior and actions as being inconsistent with their argument, so that the opponent is hypocritical. This specious reasoning is a special type of attack. The cites John Cooke's 1614 stage play  as the earliest known use of the term in the English language.

Form and explanation
The (fallacious) argument follows the template (i.e. pattern):
 * 1) Person A claims that statement $X$ is true.
 * 2) Person B asserts that A's actions or past claims are inconsistent with the truth of claim $X$.
 * 3) Therefore, $X$ is false.

As a specific example, consider the following scenario where Person A and Person B just left a store.
 * 1) Person A: "You took that item without paying for it. What you did is morally wrong!"
 * 2) *Here, $X$ is the statement: "Stealing from a store is morally wrong." Person A is asserting that statement $X$ is true.
 * 3) Person B: "So what? I remember when you once did the same thing. You didn't think it was wrong and neither is this."
 * 4) *Person B claims that Person A is a hypocrite because Person A once committed this same action.
 * 5) Person B has argued that because Person A is a hypocrite, he does not have a right to pass sentences on others before judging himself.

Other artificial examples
The example above was worded in a way to make it amenable to the template given above. However, in colloquial language, the technique more often makes an appearance in more subtle and less explicit ways, such as in the following example in which Person B is driving a car with Person A as a passenger: Although neither Person A nor Person B explicitly state what $X$ is, because of the colloquial nature of the conversation, it is nevertheless understood that statement $X$ is something like: "Running stop signs is wrong" or some other statement that is similar in spirit.
 * 1) Person A: "Stop running so many stop signs."
 * 2) Person B: "You run them all the time!"

Person A and/or Person B are also allowed to be groups of individuals (e.g. organizations, such as corporations, governments, or political parties) rather than individual people. For example, Persons A and B might be governments such as those of the United States and the former Soviet Union, which is the situation that led to the term "whataboutism" with the "And you are lynching Negroes" argument.

The technique can also appear outside of conversations. For example, it is possible for someone who supports a certain Politician B, who recently did something wrong, to justify not changing their support to another politician by reasoning with themselves:
 * "Yes, Politician B did do this-or-that immoral thing, but then again so do other politicians. So what's the big deal?"

In this example, Person B was "Politician B" while Person A was "other politicians."

Whataboutism is one particularly well-known modern instance of this technique.