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Tear Jerker: Transformers Cybertron
Transformers Cybertron with Vector Prime disappearing after reversing time to let the Autobots fight Galvatron after the space bridge at Gigantion was destroyed.
What really hammered this home for this troper wasn't so much the act... as it was the youngest of the kids being so deep in denial and grief that he couldn't even comprehend why everyone else was crying their eyes out at first, and then he completely breaks down too.