This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Secret Life Of A Mermaid
In the climax of "Birthday Splash" is rather heartbreaking, Amy gets sprayed with water, has to wait hours to finally get dry, and returns to her party, only to discover that all the guests left and that all of her cookie cake is eaten (Brenna saves her a "piece", which is really just a bite size scrap). Basically her guests and her own freaking best friend started her party without even bothering to wait for her, causes her to break down into tears.
This episode also contains elements of Harsher in Hindsight when Kelsey tells Amy her dog died on her birthday. Christine and Bridget's dog Virginia who had been seen regularly on the show had died recently.
Blaire, anyone? She suffers from Parental Abandonment due to her mother leaving her and her father because she couldn't live on land and her father hardly ever spending time with her because he's always fishing, and her only friends sometimes use her as a tool for mermaid information. She accompanies these tragic events with an innocent smile and wide blue eyes
The end of Season 4 is a major Tear Jerker for Amy, Brenna, Kelsey and Blaire fans