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Tear Jerker: K-On! aka: K-On
The end of episode 20 of the second season. The girls just sit under the windows of their club room late in the day after their last live, and acknowledge they will no longer be a band after they graduate. Any form of sadness that the second season held behind a dam underneath the cheery, cute, colorful exterior comes falling down. The impact of this scene loses weight when all four seniors later pass the entrance exams so they go to the same university together, but it's still dramatic.
Yui: ...and our next senior festival will be even... better...
Add to this that, during that scene, the BGM is a slower instrumental version of U & I, something of a love song that they had just played as their final number.
Episode 24 of the second season. Especially because of the lyrics of the song the club sings at the end.
Especially the last two verses:
At the train station, on the path by the river, no matter how far we are, we can look up to the same sky and sing as one!
But you know, I met a wonderful angel. Graduation isn't the end, because we're best friends from now on.
If you say "I LOVE YOU!", we'll reply "WE REALLY REALLY LOVE YOU!!" We won't lose anything, forever and ever we'll always be together.
The lyrics to U&I. It's obvious Yui is talking about her little sister Ui and how she doesn't know what to do without her. It's mostly cute and moe ("Where's the soy sauce and the sugar? Without you I can't do anything." etc.), but near the end of the song, it seems Yui has finally realized how important her little sister is to her, and how they won't be able to be together much longer:
I'm so sorry. I only now just realized, I've been taking you for granted. So, to start with, I have to tell you, thank you so much!
And the seiyuu also teared up a bit at the end of their Real Life concert! Kyo Ani probably took some cues...
According to the same source, Bice passed away July 26. EP 17 debuted July 27: the NEXT DAY... how rough is that?!? (TT- TT )...
This video of the final insert song, Tenshi ni Fureta yo! showcases the memories Azusa shared with each of her sempais, in sync of each their individual vocals during the song, concluding with the most memorable moments they shared as one when all four sing together.
The original scene where they play Tenshi ni Fureta yo! counts. Just look at the comments on the link to the original above...