The two wrongs fallacy occurs when an accusation of wrongdoing (or other defect) is answered by pointing out similar wrongdoings or defects. The problem is that there being some additional instances of something wrong is not relevant to whether the original case is itself wrong.

Here is an example:

Person A: "You shouldn't roll through a stop sign without coming to a complete stop."

Person B: "I've seen other cars do it at this intersection, so I think it is OK."

Person B overlooks that other cars failing to stop only worsens the problem, and does not excuse their behavior.