Regular Show/Tear Jerker

"I can fix this" is a hilarious line until Rigby says it in reference to Mordecai's "death" (he had actually passed out due to hypothermia), completely out of desperation and very close to just full-force sobbing. The scene only lasts about a minute, and it's still completely heartbreaking.
- Also in that episode, when Mordecai finally reaches his limit, he lets loose a surprisingly convincing rant about how Rigby is always messing things up for him. It sounds just like a big brother telling his little brother to get lost and find his own friends, which is pretty hurtful.
  - And he nearly died hating Rigby.

Rigby: Don't die hating me!!

Rigby's death in "It's Time" is also considered a tear jerker, especially when Mordecai recalls his last moments with him.
Leon's Heroic Sacrifice in "Benson Be Gone".
Rigby dying a second time in "Appreciation Day", although less so than his other deaths.
Rigby dying a third time in "Over the Top".
The desperation in Mordecai's voice as he's trying to stop Rigby from attempting to un-jinx himself.
"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 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 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 tell anyone for help 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.
Benson's flashback in "Stick Hockey". His apprentice and friend, Dave, was killed right in front of him in one of the worst ways possible.
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 rather unsettling.
- 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.
"Eggscellent". Just, "Eggscellent". I was not expecting Rigby being put in a near death coma and Mordecai trying to win his the hat he wanted in the omelet eating contest. The biggest sucker punch would be Mordecai punching Benson after he says "This is what happens when you slack off duty" and telling him they're only his friends because he's their boss. Most of the episode was almost like a more dark take on the duo's regular activities, especially if you were spoiled early by the official description of "Over the Top".
"Busted Cart". Holy hell. After Mordecai and Rigby's typical shenanigans with the golf cart put Benson through a LOT of 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.