Benson. As the series progresses, we learn more and more snippets of his backstory. And man was his life sad. Turns out he's not the rage-filled, unreasonable, asshole of a boss that we all thought he was.
Then there is this scene◊ that was never animated from "Mordecai and Rigby's"" which reveals that Benson had a girlfriend who left him for a business man and told him that he would never make it in the music business.
Any time Pops gets upset. He's such a sweet guy, and he acts like a little kid. He's quite sensitive.
"Meat Your Maker" is chock full of awful moments. The entire freezer thing from Mordecai's growing annoyance to Rigby's increasingly desperate attempts to appease him to the whole near-death experience is pretty gut-wrenching. And then there's the fact that Benson was really looking forward to the barbecue and put a lot of money and effort into it, only to take the blame for everything that went wrong when it wasn't even remotely his fault.
Don. The relationship between Rigby and his brother. Sad from Rigby's perspective- anyone who has felt oveshadowed by a sibling can relate. And gut-wrenching from Don's perspective. All he is is nice to people, and he has no idea why Rigby despises him so much. Also doubles as heartwarming, when Don tells Rigby that he'd actually been trying to act just like his older brother the whole time.
Rigby's death in "It's Time" is also considered a tear jerker, especially when Mordecai recalls his last moments with him.
"Skips Strikes" had Skips quitting the bowling team and the park members (with Pops replacing Skips as a team member) listening to some sad music while knowing that if they would lose to their opponents, Death from a previous episode would take their souls and what's worse, Skips quit because of his real name, Walks, being an embarrassing name.
As for Skips, Skips sadly burning the championship items he had and going through the memories of when he and his friends first joined the team is heartwrenching enough as it is.
The music that played during Muscle Man's suicide and the burning of the audience in "Terror Tales of the Park".
YMMV, but I found all of Rigby's friends dying to protect him from the wizard tragic.
In "House Rules", Old Mordecai implies that Rigby dies young. It's a quick one, but it's hard to laugh at the rest of the episode after hearing it.
What about how Mordecai feels about this? He's been best friends with Rigby since they were little. So to learn that your long time best friend may die young...you can tell by the tone of his voice when he asks 'how is this happening' that he was so upset! He can't tell Rigby about it cause that would be like telling your kid sibling they're going to die young! And Mordecai can't call for help from anyone like Skips or even Benson cause he doesn't trust anyone enough not to let it slip to Rigby. So he's suffering this knowledge alone.
In "Think Positive", Benson was forced by Pops, of all people to hold all of his anger in. He buys an anger management tape, which triggers a flashback of his family. Apparently his family yelled a lot, and that's where his Hair-Trigger Temper stems from. It's sort of Played for Laughs, but children who can relate to such unhealthy family dynamics may find this part saddening.
At the end, after struggling desperately not to yell the entire episode, he floats in the air dramatically, with metal and trees rushing towards him and his fiery force field thing, as if he were a vacuum until he finally does let ALL of his anger out at Mordecai and Rigby, and says nothing positive about them. Not only that, but there were TEARS as he did so. And the rant itself was hard to listen to. May double as Nightmare Fuel and/or a Moment of Awesome.
"Busted Cart". Holy hell. After Mordecai and Rigby's typical shenanigans with the golf cart put Benson through a LOTof crap (getting yelled at by his boss and being forced to drive across the country to transport the cart to the dealership in a day before the warranty expires), Benson finally yells at the two about the consequences of their actions after they waste a lot of time in a roadside arcade overnight instead of waking him up so he can continue driving to the dealership. It's just another typical Benson rant... until he breaks down in tears midway through his speech and sobs about how his job is the only thing he's got.Damn.See here.
Benson: (to Mordecai and Rigby) Excuses, excuses! HOW AM I SUPPOSED TO TRUST YOU WHEN ALL YOU GIVE ME ARE EXCUSES?! WHEN ARE YOU TWO GONNA LEARN THAT YOUR ACTIONS HAVE CONSEQUENCES! CONSEQUENCES THAT AFFECT OTHER PEOPLE! *suddenly looks sad* Like me... Don't you two understand? *eyes well up with tears* I'm about to lose my job! You may not care about keeping your jobs but I care about keeping MINE... *tears stream down his face* because if I lose my job... I have nothing! Y-YOU HEAR ME?! I have nothing! *buries face into hands and sobs* Now if you'll excuse me, I'm going to take my last bathroom break as an employed man.
"Trucker Hall of Fame". Muscle Man's dad died. At the funeral, he has a breakdown. Even High-Five Ghost is in tears. Just thinking about it puts Muscle Man in tears. When he was driving to spread his father's ashes, he reminisces about Dad's fun pranks. Mordecai and Rigby see Muscle Man's rivals and they repeatedly tease him (and remember at the end they wanted to kill the gang just because they hate Muscle Dad.). He's in a field and smashes a photo of Dad and has another emotional breakdown, and then finds a letter from his dad. Ugh, this troper wants to shoot the guys who bullied Muscle Man with a Berretta loaded with infinity bullets. Don't you wanna give Muscle Man a hug?
The ghost of Muscle Dad returns, though. And he was a forklift driver, not a trucker.
The beginning of the episode is just heartwrenching. Muscle Man is listening for a signal from his dad. He only hears static, and he laughs, assuming that his dad is pranking him. Then Muscle Man's brother walks in, looking forlorn, holding his father's hat... That look Muscle Man has on his face. Jesus Christ.
"You know who 'else' is really sad my dad died? MY MOM!" *sobs*
"Out of Commission". It starts off fine... until Benson announces that the old cart they had for quite a long time will be replaced. Mordecai and Rigby poured energy soda on the cart that drips into the engine—causing the cart to become alive. Seeing as they would cry if they had to decompose it, they take him out, for one last spin. At the end, the cart announces he wants to commit suicide by drowning in an ocean, as well as exploding. Driven to Suicide taken a literal (yet different) spin.
During the events of "Diary", we learn why Skips always skips: many years ago, he would skip with a woman he loved. After she passed away, he vowed to always skip from then on, so that he would never forget her.
This episode also proves that Mordecai does, in fact, still have memory of Rigby dying in "It's Time."
In "Prankless", Muscle Man is making a "bed on the ceiling" prank when it accidentally falls down on and ALMOST KILLS POPS. With all of his sadness, grief and guilt, he quits his job and refuses to prank.
Poor Pops. Pops ran into the room, happily chasing a balloon. The balloon tapped the bed, which was taped to the ceiling... and the bed fell on him. Pops cries loud enough for all the park staff to come to his aid, and when Skips lifts up the bed they find Pops crushed and severely injured, and still crying loudly. Muscle Man just stood on the ladder with a look of horror on his face. Then Pops is taken away in an ambulance, moaning in pain.
The episode gets worse when you add in the Fridge Horror; Muscle Man already lost his father this season by a tragic pranking accident. He almost lost Pops, another term for dad, by a prank set up by him. No wonder he was so quick to quit.
Thomas in "Exit 9B" and "Starter Pack". He gets put through crap just to earn respect and work (for free even) but it is lightened how laid back he is and he is determined to get his work done.
Mordecai's story in "Terror Tales of the Park II" can be a little painful to watch to those who have lost loved ones. His uncle dies in the story, and he is haunted by his ghost. Mordecai's terror is just pitiful to watch.
"Caveman" - when Gregg the caveman decides he's not fit for the modern world, he refreezes himself, his mate, and his caveman brothers while giving Mordecai and Rigby the thumbs up they showed him. The two weep as he is frozen, and even Benson cries after seeing Gregg gave up a new life for love.
"That's My Television." RGB 2's situation was a great deal an overlap between this and Nightmare Fuel.
From "A Bunch Of Full Grown Geese." Pops getting attacked by geese, and eventually he's so terrified and distressed that he actually locks himself in his room and cries. Poor thing.
"Steak Me Amadeus", Mordecai plans the perfect date in order to ask Margaret to be his girlfriend, but Margaret reveals she's been accepted to her dream school and can't be his girlfriend, followed by her running out of the restaurant in tears. The final scene with Mordecai and Rigby on the roof hits you hard.
Eileen's sadness with seeing the president of Tants burning the fake Tants that she worked hard on with Mordecai and Rigby.
The Thanksgiving episode has a few, chief among them are Mordecai and Rigby's song at the Turducken Contest due to Their families not being able to come for Thanksgiving, later revealed to be Thomas thinking their planes were delayed.