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Tear Jerker: Outlaw Star
The episode "Cats and Girls and Spaceships" from Outlaw Star, where Jim unknowingly kills the girl he had liked, and when she understandably fails to show up for their date afterward, he assumes she stood him up and never finds out the truth. Making it even worse. The girl has no idea she was fighting the boy she loved, and her last thought is she'll never be able to see Jim again.