Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Tear Jerker: Return of the Living Dead
In the first film:
Frank's suicide. The earnestness and sincerity of the scene comes from out of nowhere for some Mood Whiplash, as he kisses his wedding ring, then ceremoniously gets in the crematorium oven to kill himself before he can turn.
Freddy's inability to stave off his zombie transformation is both funny and sad.
"See? And now you made me hurt myself again! You made me break my hand completely off this time, Tina! But I don't care, Darlin', because I love you, and you've got to let me EAT YOUR BRAAAAAAAAAAAIIIIIIIIIIIINS!"
The line would be even more funny, but Tina's real anguish and sobbing makes the line more pathetic and sad than hysterical.