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Tear Jerker: School Rumble
Chapters 273 of the manga. Not to mention the chapter RIGHT AFTER that, where Tenma comes to see Karasuma and he lost all memories of her, while she tearfully recounts all the good times they had during school. Absolute heartbreak, especially when Tenma starts to cry.
Parodied when Madoka, Shigeo and Mai shoot each other to the tune of "Feel Like A Girl". Then again, they were just playing...
Kind of a Fridge Tearjerker, the scene early on where Tenma overhears Harima's story about rescuing her from hoodlums and botching his attempted Rescue Romance. Tenma, having no memory of the incident (or at least not enough to connect the dots), reassures him that it was all the girl's fault. As funny as the scene is it at first, it does leave you wondering whether Tenma had regretted rejecting him the way she did.
Eri and Yakumo's friendship falling apart after Eri started to ostracize Tenma whom she thought had feelings for Harima.