Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
The fallout from Anna's death, especially when it comes to Mal and Cameron's relationship, which is pretty much ruined.
Eiryss/Fire meeting Cameron. She tried to remind herself that she's going by Eiryss and she's not related to him but she failed. Worse, seeing the girl who was just a few moment ago a badass capable of taking down her own Alternate reduced to a little girl clinging to her father was just ... heart breaking.
Both versions of Fire in general are this, to be honest.