Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Tear Jerker: Ink City
Towards the end of the infamous Benson/Mordecai/Heloise Log, aka Log #3, Benson squeezing Mordecai's hand as a last gesture of comfort before Hely smashes his head in with a hammer.
The utter breaking of Rigby during Event #1, leading up to the point where, after witnessing Chipotle and Phoenix getting sucked into the Mayor's black hole, he chooses to jump in. Benson's decision to join him is just as depressing.
The end of Log #36, when Jimmy and Hely say their goodbyes as he melts away.