This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Boku Wa Tomodachi Ga Sukanai
In the fifth episode of the second season, Rika gives Kodaka a chance to try her new invention that allows people to dream whatever they want. Kodaka uses it to go back 10 years and tell Sora that he was moving, which he never got the chance to do. During this, however, his own voice interjects which lays out his guilt over forgetting his Only Friend and not recognizing her when he got back. Granted, what follows after that is truly unholy Mood Whiplash, but until then...
Kid!Kodaka:Yozora, wait! Sora: [shocked] How do you know what my real name is? Kid!Kodaka: Listen! My family and I are moving day after tomorrow. I won't be back for around ten years. So that means tomorrow will be my last chance to see you! Sora: Really? Kid!Kodaka: I'm sorry! Present!Kodaka:For leaving you alone. Kid!Kodaka: But we'll stay friends, even if we don't see each other anymore! Present!Kodaka:Except you forgot about her until you came back. Kid!Kodaka: No matter what, I promise I'll never forget you! Present!Kodaka:And then you couldn't even place her face once you did. Kid!Kodaka: I'm sorry. I really am. Present!Kodaka:And I always will be. Very sorry.