This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Lady and the Tramp
The dogs howling the song "Home! Sweet Home!" during the dog pound sequence, courtesy of The Mellomen.
During the scenes in the pound, we see a dog being led off to another room, where it's heavily implied the dog was put to sleep. They never outright say it, but Lady turns to the other dogs and says "You...you mean he's...?"
Trusty's Disney Death. Jock whimpering, then when he nudges Trusty and there's no response, he looks up at the sky and lets out a mournful howl.
It would've been even worse had the filmmakers gone with the original plan to kill him off for real. Fortunately, they decided against it.
Makes it doubly more saddening when Tramp leaves (because Lady tells him to) and you can see from his expression that he really fell in love with her.
Lady's sad and confused reaction after Darling hit her. It wasn't anywhere close to animal abuse, but you can't help but feel pity for the poor girl.
There's Lady's reaction to Jim Dear calling her "that dog". Jock and Trusty couldn't believe it when she told them about it, and she said that he'd never called her that before.
Lady at the pound after learning of the Tramp's womanizing ways. She just let herself be romanced by him and actually fell in love with him. Then she thinks his feelings weren't genuine. She probably feels like an idiot. Further emphasized when Jock and Trusty offer to marry her, implying Tramp might have impregnated her as well.
The puppies born at the end confirm this to be true. What if she gave birth and Tramp had left her... ouch...