Fridge Sorrow

On the surface, something is light-hearted, but take a closer look and it becomes really depressing. This can range from a Tear Jerker to something outright completely miserable to think about. Usually. though, since it is a Fridge Logic event, the overall tone otherwise is so upbeat that it's hard to spot.

Web Comics

 * Homestuck is very light hearted and is almost always making jokes, but the more you think about it, the more depressing it gets. A group of 13-year-olds accidently destroyed their planet, killed all other humans, see their guardians' dead bodies, and have to die in order to achieve a power level capable of winning the game. It's utterly depressing!

Web Original

 * Atop the Fourth Wall has a blend of both Fridge Brilliance and Fridge Sorrow on this one. I always wondered why Lewis never involved Liz in the show more. Linkara's a character who's incredibly prone to badass, heroic speeches, so it always seemed weird to me that he never did like a "THIS IS MY GIRLFRIEND, AND YOU WILL NOT HARM HER — DO YOU HEAR ME?!" thing to any of the villains. It was always very underplayed compared to show's usual over-the-top approach to most things. But as their eventual breakup (and some fans' extremely inappropriate reactions toward her for it) went to show, it was probably a very good idea not to mix business with personal too much.

Western Animation

 * My Little Pony Friendship Is Magic
 * It's likely that Pinkie Pie doesn't constantly throw parties just because she likes them. It's because she knows everyone else likes them, and it's the only way she knows to be sociable on a normal level. The Genki Girl randomness that she's known for is too much for most folks, who've probably actively avoided her for it in the past. But if she throws a party, suddenly everyone likes her, and she clings to that. So it's parties forever, and nothing else. Because if she stops, she stops being "That eccentric girl who throws awesome parties all the time" and reverts to "That annoying girl who talks too quickly about things that don't make sense." And then she's all alone. Again.
 * "At the Gala" is a Crowd Song, right? But the way the other ponies are singing, it implies that most of them were also hoping to have the best night ever. And most of them seemed to be succeeding a lot better than the Mane Cast before the party got ruined. That means hundreds of ponies also had their dreams destroyed in a space of about five minutes.
 * Twilight got all of her old notes back from Princess Celestia, and they just kept coming, there is a scenario that most likely have played out for Celestia herself. If she has to send the notes one by one, something she must know would be harsh on spike, then that means she couldn't just deliver the notes herself. That likely means Discord has her confined, probably in the castle, so she can't interfere. The fact that she keeps sending them means she doesn't know if they are getting to twilight, or if they would help matters any. For the first time in eons, she is helpless, clueless, and has no idea how things will turn out. But if she is so cut off from communication, how does she know twilight is in need of the notes? Discord likely stopped by to show his old rival the exact moment twilight's heart, Celestia's last hope, finally broke. So the most likely situation is Celestia, feeling powerless and blind, knowing only that Twilight's spirit is broken, desperately sending letter after letter with no clue if it will change anything. Those may very well have been some of the most stressful and heartbreaking hours she has experienced in her long life.
 * The scene where Twilight is trying to find the book about the Elements of Harmony, but her friends keep interfering her due to their corruption. It is, in a way, a rather accurate portrayal of bullying: what bully victim wouldn't relate to Twilight, whose own friends are bothering her for absolutely no reason? Who wouldn't be in a completely irritated mood, and on the verge of giving up? And think from the other ponies' perspective, the bullies' perspective. One day, you might be friends, possibly best friends with someone, but out of some internal and/or external force (think peer pressure and insecurity instead of Mind Rape), you find yourself to be the very person you hated. Then you start bullying your former friends; and it doesn't help that bully victims are often socially awkward "nerds" like Twilight. Fluttershy's reaction after being restored pretty much confirms this.
 * Think about Twilight's doll, Smarty Pants. When you consider that Twilight has such little knowledge of friendship, combined with the scenes in Lesson Zero of fillies laughing at her, it's quite possible that Smarty Pants was the closest thing Twilight had to a friend growing up.
 * New!Fluttershy saying that Rarity and Pinkie Pie are wasting their lives with fashion and parties and nopony else gives a flying feather about them is a lot more depressing when you realize that those are the things that, as Twist would say, makes them special. She basically told them their lives are meaningless. That's harsh. Is it any wonder they ran away crying.
 * Codename: Kids Next Door
 * More like a Fridge Tear Jerker, but Leona is 350 years old and is still physically and probably mentally 10. Everybody she knew and loved is looong dead. A (technically) little girl outlived all her relatives.
 * And if her family did live 350 years ago (with all the diseases and war and such), their deaths probably weren't that pleasant.
 * More Fridge Tearjerker—and this may seem obvious, but I didn't pick up on it when I was a kid; when Leona and Numbuh 5 hug each other in the bathroom, Leona lets herself age, 5 asks "Are you sure you're gonna be okay?"...We just watched someone decide to die, and 5 asking whether Leona is okay with death.