This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Jonathan Creek
Maddy watching the destruction of Gordon Hill, the neighbourhood where her mother killed herself.
The revelation in Satan's Chimney that Tom was tricked into murdering his own mother.
Polly is haunted by dreams of the childhood figure she always knew as the Sandman and saw as a malevolent figure in her childhood in The Sinner and the Sandman. Come the end of the episode, we learn that the Sandman was just a vet who had to come and put her beloved pet rabbit to sleep. There are carrots at the bottom of her dad's garden- it turns out that these mark the spot where the rabbit was buried, and the carrots were planted so that the rabbit would always have a snack. Polly's dad kept planting these carrots regularly until his death. Sob.