Sayonara Zetsubou Sensei (2007) : 1x9

Nozomu notices a few members of the class playing a "spot the differences" game. Nozomu tells them that it is important to spot mistakes and he instructs the class in how life is filled with mistakes. He asks the class to identify mistakes in their lives after pointing out several of his own. After Nozomu contradicts himself, saying that a mistake was the correct action, Kafuka reasons that if a mistake comes to light, it is correct, resulting in many people approaching her, confessing their mistakes, making them correct. In the end of the first part, a boy approaches Nozomu, calling him "Dad." The second part of the episode reveals that he is actually Nozomu's brother's child, Majiru. Nozomu asks him to prove that he is indeed his nephew, and then asks all of his students to prove that they are the real one. When Majiru asks Nozomu to prove he is genuine, Nozomu is unable to come up with a response, and runs off. The episode ends with a sequence that deals with Majiru's hair fetish.