Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Tear Jerker: Super Robot Monkey Team Hyperforce Go!
The entirety of the third act of the Season Two finale.
Antauri's death at the hands of Mandarin.
Monkeyman!Chiro attempting to bring Antauri back.
Watching Sprx have a break down as he watches Nova get sucked into a wormhole, listening to her cries for help when he can't do anything to save her, is absolutely heartbreaking.
Made even worse when you realize he'd been trying to confess his feelings to her not a scene earlier. He'd been this close to finally telling her, and then she literally slips through his fingers, presumably forever.
Sprx turning evil could be considered this to some people, too.
Watching Sprx electrocuting Nova while also threatning to magnetically tear her apart if the Monkey Team so much as twitches.