The ending of Noir, when wounded and battered Mireille helps even more damaged Kirika walk away from the Manor, suggesting they have tea when they get home. The scene is so heartwarming, even the two off-camera shots right afterward couldn't convince most fans that it was supposed to be Downer Ending. To be fair, they were shots from the girls' trademark guns (sound analysis, and the fact the studio used recordings of actual gunshots by the respective guns to render the sound effects, prove it) makes it highly unlikely it was meant to be in the same context as the scene immediately preceding it..
What about Kirika's love letter to Mireille? Slightly TearJerkery too.
For this troper it's the beginning (when Mireille is trying to force herself to shoot a wounded Kirika) and end scenes (when she talks of the "black thread" that binds them together) of "The Black Thread of Fate".