Absurdly High-Stakes Game: "Skips Strikes" has Rigby making a bet with Death: If Death's Team wins he'll get the team's (which is everyone working at the park except Muscle Man and High-Five Ghost) souls and if Rigby's Team wins he gets... a bowling ball filled with souls. Mordecai and Benson aren't happy about this in the slightest.
"Over the Top" has Skips arm wrestling Death for Rigby's soul.
In "Slam Dunk", Mordecai eventually bets his computer privileges for life on a basketball game just so he can help Margaret make a website in hopes of impressing her.
Adrenaline Makeover: Well, somewhat, in that Mordecai and Rigby give themselves mullets while learning Death-Kwon-Do.
An Aesop: From the episode "Mordecai and the Rigbys", Mordecai and Rigby noticed at the end of the show that their future selves stayed famous through lip-syncing. Mordecai makes a speech about the issue.
Also, in "Cool Bikes", Mordecai told Benson (after countless hours trying to impress him) "We don't care what you think."
In "Steak Me Amadeus", the villain says, "I didn't go to school! And look at me!" and then violently explodes immediately after.
"Video Game Wizard" where Rigby gets jealous that Mordecai chose Skips as his partner for the video game tournament is an affectionate parody of The Wizard. It even has a crappy power glove knockoff as the grand prize.
Alien Geometries: In "Brain Eraser", the world of Mordecai's memories is a strange place.
Skips: Memory isn't logical.
All Adult Animation Is South Park: Whilst it may be on Cartoon Network, the show was originally and still is created for adults, except that Adult Swim shipped it off to Cartoon Network instead. J.G. has said in an inteview that he is trying to be as edgy and as near the knuckle as possible.
Also their combined efforts in defeating the physical manifestation of "Summertime Lovin'."
In "A Bunch of Baby Ducks", the baby ducks fuse into a giant duck person to defeat the creepy guy with the van.
And in the hundredth episode "A Bunch of Full Grown Geese", the baby ducks fuse with Mordecai, Rigby, and all the products they had collected for the past 99 episodes to defeat a King Ghidorah-esque fusion monster of grown geese.
Aloof Big Brother: Don who is taller and more mature than Rigby actually subverts this, since he is actually younger than Rigby, as well as extremely affectionate towards his sibling. Rigby was jealous and angry that his friends liked Don more and that they think he is the older brother, so he rejected Don's affection and hated having to see him. Which makes Rigby the Aloof Big Brother!
And I Must Scream: Apparently viral videos have their stars trapped inside of them. They're forced to repeat their injuries and mistakes over and over again. Until our heroes come to their rescue, of course.
Angst Nuke: When Benson finally releases all his pent up anger in "Think Positive" the results are explosive.
Arson, Murder, and Jaywalking: In "Dead at Eight", Death's refrigerator is populated by such abominable foodstuffs as eyes, Death Nuggets, Soul Juice, and almond milk.
Artistic License: Mordecai and Rigby don't resemble the real life animals they are supposed to be. This is because of needing to be human sized and also Rule of Cute. Rigby especially looks more like a ferret or a squirrel than a raccoon. Mordecai doesn't seem particularly interested in flying despite being shown to do so in on rare occasions. This is lampshaded in the episode 'Dizzy', where they cross over into a bizarro world in Pops's mind, with photorealistic versions of themselves who try to attack them.
Artistic License - Biology: In "Laundry Woes", Mordecai's feathers are treated as hair, with him growing a beard and shaving it off.
Artistic License - Physics: In "Go Viral", somehow, jumping on a trampoline at the same time a refrigerator is dropped onto said trampoline will cause you to bounce thousands of feet into the air.
Art Shift: The episode "First Day" consists mostly of animation from the unaired pilot, with subtle but noticeable differences in animation style.
When Power Tower first aired, it had used a art style of crayon coloration.
Bad Boss: While Mordecai and Rigby are by no means decent employees most of the time, neither is Benson a particularly good boss. He barely ever trains the duo for the tasks he asks done, then berates them for not doing what he wanted how he wanted; he rarely gives positive reinforcement, ofttimes going out of his way to single them out; he continually doles out expectations that by all means he knows perfectly well can't be met; combine with a chip on his shoulder and blatant favoritism, and it shouldn't be any surprise that he gets no results from the slackers.
As Mordecai and Rigby's Fatal Flaw is that they are Brilliant but Lazy, Benson’s is that he believes a Hair-Trigger Temper is an acceptable form to lead people. When Pops makes the completely reasonable petition to stop yelling at Mordecai and Rigby, Benson discovers he literally cannot do that.
Mordecai and Rigby seemed to be flanderized in that episode to make it absolutely impossible for Benson not to yell at them.
Mr. Maellard is way worse than Benson. He generally gets angry for no reason and treats Benson like a butler who doesn't work right, which probably is the reason Benson is such a angry boss. He's taking in everything Mr Maellard throws at him and takes it out on Mordecai and Rigby.
Bait and Switch: In "The Christmas Special", Muscle Man comments on the cookies, and then asks "You know who else makes great cookies? MY-chael's (Michael's) Bakery on Fifth and Western. You ever been there?"
Balloon Belly: Party Pete after drinking several Radicolas which gave him strange powers. After finding out Benson was going to return to the park to see what all the commotion was about (They were having a party without Benson's say so), The gang had to keep giving Party Pete Radicolas so he'll burn out in order to get rid of him before Benson showed up, thus ending Mordecai and Rigby's party with a bang.
Band of Brothers: While the park employees take the piss out of each other for fun, and get on their friends' nerves more often then not, they have definitely become this.
Balloonacy: Happens to Pops in "Just Set Up the Chairs"
Big Damn Kiss: Near the end of "Picking up Margaret", Mordecai and Margaret finally kiss.
An even bigger one happens in "Meteor Moves".
Bilingual Bonus: In the opening of "Fortune Cookie", when everyone is at the Chinese restaurant, we get a couple of shots of the Dim Sum receipt. It has four rows, labeled with the Pinyin characters for "small", "medium", "large", and "king".
In "Fool Me Twice", "Baka" in "Baka Blitz" translates roughly to "Idiot".
Big "NO!": The Death Kwon Do teacher lets out three of them in the course of one episode.
The Master Prank Caller gets two.
GBF gets one.
Susan gets one after going One-Winged Angel and Benson gets everyone to stop working.
The Night Owl gets one after the guys make it back to the time machine and escape him. The present Night Owl gets one when they come out of the portal crashing down on his billboard and car, ruining his plan.
Muscle Man gets one when he realizes his dad was never a truck driver.
Big "Shut Up!": Rigby, very often. A variation of this that he also uses very often is, "STOP TALKING!!!"
Birthday Episode: The episode "Free Cake" revolves around Mordecai and Rigby setting up a birthday party for Skips, but only so they could have free chocolate cake. In trying to get Skips to the party, they interrupt a ritual that gives him eternal life, and have to sacrifice the cake to save him.
Bishie Sparkle: Starla sees Mordecai with these when she falls for him.
Body Horror: What happens when you work under Susan for too long.
A good example of this trope is when Rigby physically turns into a house. The slow transformation doesn't look comfortable either.
Bowdlerization: The international airings of this show are either edited to pieces or have missing episodes. The biggest offenders are the versions shown in the United Kingdom, Australia, Canada, Southeast Asia (particularly the version shown in the Philippines), and Latin America.
Bowdlerise: The two instances of Mordecai saying "Pissed"(From "Free Cake" and "Meat Your Maker") have been dubbed over with "Ticked" in recent airings.
From the pilot: "Rock-paper-scissors is an evil game!"
Mordecai seems to really like chick flicks.
Near the beginning of "150 Piece Kit" Rigby mentions that he knew a guy who tried to perform the legendary drum solo that the episode is centered on and his skeleton caught on fire. Benson suffers similarly during the episode's climax but survives.
Margaret said she was going to use Mordecai's butt dial song as her ringtone, and she does.
In "Really Real Wrestling", Pops accidentally crushes Huge-Head with his flying car. Later, Pops flies home with a strip of police tape stuck to one of the car's wheels.
In "Dead at Eight", Death mentions that Muscle Man would die in a hot dog eating contest. This doesn't get brought up again until a whole season later, in "Last Meal". Death did join the contest himself just so that he could take Muscle Man's soul if he won, however, so it's more of a self-fulfilling prophecy. Muscle Man wins anyway.
A bet was made at Halloween that the person voted to have the lamest story would have to wear their costume for a whole month. Thomas was finally able to take his pizza slice costume off at Thanksgiving. He desperately needed a shower.
Broke Episode: Both "Free Cake" and "Caffeinated Concert Tickets" qualify.
To a lesser extent "The Power", which involves asking for raises to... hire someone else to fix a hole in a wall.
Brutal Honesty: In "Pie Contest", Mordecai and Rigby are pie judges but they don't want tell everybody how bad their pies are to avoid being hated. But they have to do this when a talking pie starts eating everybody and the only way to stop the pie and save everyone is to tell them the truth.
Bullying a Dragon: In "Last Meal", after Death loses the hot dog eating contest, Marty, the hot dog vendor, has the nerve to call Death a loser and gives him a complimentary T-shirt for losing the contest. Death told him to enjoy the next few days because his deadline just moved up big time.
Caffeine Bullet Time: Eating Double-Glazed fritters sends Pops into such a huge Sugar rush, that Mordecai, Rigby, and Skips consume one each as well in hopes of making all 4 of them suffer from a sugar crash. It doesn't work, all it does it make them move so fast that the rest of the world started slowing down. Eating even MORE sugar only made things worse, starting with colors and physics starting to break down.
Call Back: In "Grave Sights", Benson asks Skips to set up the chairs, making an obvious callback to "Just Set Up the Chairs".
In "Benson Be Gone", Maellard complains to Benson about snacks disappearing from the snack bar, holes in the walls, and five hundred dollars worth of prank phone calls, which are all references to the episodes "Rigby's Body", "The Power", and "Prank Callers" respectively.
In "Camping Can Be Cool" Mordecai and Rigby bring the Super Extra Premium Hot Dogs from "Meat Your Maker" to eat on the camping trip.
"Stick Hockey" seems to be a whole episode callback to a scene in "Benson Be Gone", in which Benson starts warming up to the Mordecai and Rigby over the Stick Hockey game.
The Unicorns are back in "House Rules", prank calls are also referenced.
Mordecai also threatens to ruin No Rules Man's only good eye with a laser pointer. Everybody knows they aren't regulation.
Subtly, when payday is first shown in "High Score", Mordecai complains about how they're essentially receiving doggy bags full of change, and ask to at least get an envelope. Later, in "Temp Check" they receive their pay in an envelope.
In "The Power", one of the things Rigby sends to the moon during the Cutaway Gag is "a bunch of baby ducks". There's a later episode with that exact name.
The Ferguson Convention Center from "Video Game Wizard" is named after Garret Bobby Ferguson, from the episode "High Score". A bust of him is even seen outside the building.
In "The Best Burger in the World", the burger truck crashes into the Crash Pit from "Terror Tales of the Park".
During "Diary" as Rigby and Mordecai prepare to use a microwave to melt the glue off Margaret's diary, Mordecai has a flashback of the episode "It's Time" and quickly decides against it.
In "Death Bear", Eileen suggests watching "Cease and Decease" before Mordecai talks about Rigby's fear of horror movies, such as "Breakfast Slaughter". Rigby is also scared of films like "Ello Gov'nor".
The Christmas Special had the members of the park using their Hidden Depths to overcome the booby traps on the way to the lava pit- Benson used his stick-handling skills from "Stick Hockey" to beat the pinball machine and Pops used his wrestling skills from "Really Real Wrestling" to pin the polar bear.
Also, Skips tells Santa he'll call him to go bowling sometime, like he did with other magical beings in "Skips Strikes".
In "TGI Tuesday", among the food that Mordecai, Rigby, and Eileen brought to the party are hummus and mini toast like in "Party Pete".
Rigby's refusal to hug Mordecai in "Temp Check" is possibly a call back to the episode "Don". Rigby doesn't like hugging people because it reminds him of his brother Don, who has a habit of doing it.
All the Death Kwon Do moves (except for the Death Dump) that Mordecai and Rigby use from "Death Punchies" are used again along with some new moves in "Sandwich of Death".
"Quips" features Skips and Rigby talking about the time Rigby's brother Don came over, and how Rigby tried to get rid of him but it was of no use since everybody else loved him.
In "Fool Me Twice", Mordecai brings up his experiences in art school which he mentioned in "Camping Can Be Cool" which Rigby points out that he never graduated.
In "Bad Kiss", after Mordecai tells Rigby that he and Margaret kissed, Rigby says that he owes Muscle Man a lot of money. This would later be explained at the end of "Do or Diaper" when Muscle Man bets Rigby that Mordecai and Margaret will never kiss.
"Last Meal" has Death showing up after Muscleman enters a Hot Dog Contest, and even has a flashback to the end of "Dead by 8", where he tells Muscleman, Mordecai, and Rigby that Muscleman will die in a hotdog eating contest.
The animatronics from "Fuzzy Dice" come back for revenge during "Steak Me Amadeus".
"A Bunch of Full Grown Geese" not only features the return of the baby ducks Mordecai and Rigby cared for in "A Bunch of Baby Ducks", but also a bunch of past MacGuffins such as the Maximum Glove, the PlayCo Armboy, and Rigby's "Eggcellent" hat.
Can't Get Away with Nuthin' : The entire plot of Regular Show is made up of this. The show starts out with something normal. Set up these chairs, go buy me a grilled cheese sandwich, learn to play guitar by the end of the episode, etc. However, just due to the world they live in, this somehow turns into a video game almost destroying the world, having to stop a black hole, and your future selves destroying your concert, just to name the outcomes of these examples. And of course, by the end of the episode (excluding "Prank Callers") everything is back to normal.
Character Development: "Benson Be Gone" has Benson come to understand Mordecai and Rigby more. And it sticks, afterwards, he's shown to be noticeably more tolerant of them and doesn't fly into a rage over them as often, even going as far as to helping them in "This is My Jam". In "Jinx" he actually accepts an apology from them, where before he'd not have given them the time of day. Especially since earlier, Rigby sent him in a moment of Unstoppable Rage, which Benson didn't take out on Rigby himself. Newer episodes also have him leading the group in "down time" activities such as game night or paying for drinks for everyone at a bar, showing he's lightened up more.f*
Rigby has grown much closer to Eileen; in "Yes, Dude, Yes" he eagerly suggests going to the movies with her, while in "Do Me a Solid" he only accepted a date as a solid, and did everything possible to sabotage it. Later, it goes even further in "Diary", where he admits that she looks hot without her glasses on.
Mordecai and Margaret's relationship has also developed along the series. In the first season, she's seen hooking up with a different person in every of her appearances, while in Season 3's "Butt Dial", she sets his song about her as a ringtone. Later, in "Death Bear", they almost kiss but Rigby's prank spoils it.
Margaret was a Flat Character compared to the other recurring cast, that is until "Do Me a Solid" where she's revealed to be a down-to-earth character (the unusual circumstances don't faze her nearly as much as one would expect).
Benson, Skips, and even Muscle Man and High-Five Ghost have had a good bit of character growth over the series as well, the group becoming kinder and more friendly with Rigby and Mordecai, making some of their early scenes a bit strange to how they are now. This is especially clear with Benson, who doesn't go into a rage as often. Recent episodes even have him doing things like organizing game nights for everyone and taking everyone out to a bar with the drinks on him, showing he's lightened up considerably.
Notably, High Five Ghost did not start off as an independent character, only being seen when Muscle Man was around, and having no real speaking lines. He eventually developed his own personality, which serves as somewhat of a foil to Muscle Man's often reckless jockishness.
Rigby and Mordecai's extreme carting comes into play in "Busted Cart" when they need to drive along the most dangerous highway in the country.
Mordecai and Rigby make use of their Death-Kwon-Do training from "Death Punchies" without giving it a second thought in "The Death Sandwich". Mordecai uses the Death Block a few times, among other moves. Although, Benson was with them and also knew Death-Kwon-Do, which might have come as a result of being forced to wear cut-off jean shorts and a mullet like all practitioners must wear.
Chess with Death: Arm Wrestling with Death, in Skips' case. He challenged Death to restore Rigby back to life, whom Skips had killed in the first place. If he lost, Death would've claimed his immortal soul.
A blinking contest with Peeps, a floating eyeball monster to get him to leave in "Peeps". If they'd lost, he'd have taken all their eyes.
Mordecai and Rigby had to babysit Death's son in order to save Muscle Man from having his soul reaped. The specific condition was to have him sleeping by 8 PM when Death and his wife got home from their anniversary date.
Complexity Addiction: The Capicola Gang's plan for revenge on the park workers is printing fake Amadeus dollars (coupons to buy steaks at "Steak Me Amadeus") and sell them to Pops who would give them to the rest of the workers. When they want more cheap steaks, they would set up a meeting to exchange more Amadeus dollars and then ambush them. Their whole plan is dependent on the gang's desire for affordable steaks and apparently they didn't bother to take in account that if the park workers use these counterfeit coupons they would get trouble with the police and get arrested and won't have the chance to meet them. Rigby lampshades it:
Rigby: Dude, that's really weird. Capicola Gang Leader: Well it worked didn't it?
The Computer Is a Cheating Bastard: Taken to it's logical conclusion in 'Rage Against The TV'. Mordecai and Rigby reach a legendary Final Boss that few if any have ever made it to, and none have beaten, just when they start damaging it, it fights back by shutting off the TV, then, when they connect another it manifests in the real world and they have to fight it for real before finishing it off in the game and finally winning.
Continuity Nod: A very small one. In "Just Set Up the Chairs", Benson screams that he will never trust Mordecai and Rigby to do something as simple as setting up the chairs ever again. In "Grave Sights", he has Skips set up the chairs instead.
In "Bet to Be Blonde," a pair of "Mordecai and the Rigbys" t-shirts can be seen in Mordecai and Rigby's closet, calling back to their fake band in the episode of that name.
In "Yes, Dude, Yes" we see Mordecai finish telling CJ about the events of "High Score".
In "Fancy Restaurant", Rigby brings up the last time Starla dumped him in "Muscle Woman" and how his depression forced his work onto the two of them.
In "Bad Kiss", they try to use time travel again, and near they end, they travel through past episodes.
In "The Best VHS in the World", Mordecai and Rigby are forced to clean up their room to find a VHS. Items shown in their room include The Power (still broken in two) and the grilled cheese Rigby dropped on the floor.
(after Rigby picks up grilled cheese) "Dude, I remember this!"
In "TGI Tuesday", Margaret still uses Mordecai's song as her ringtone when Mordecai calls her from "Butt Dial".
"Sandwich of Death" brings the return of Death Kwon Do.
"Ace Balthazar Lives" brings back the guy who thinks he's a knight from "Muscle Woman".
In "Do or Diaper", Moredcai makes sure to pack gum, mints, and other things to freshen breath for his date with Margret. Why? Because he doesn't want a repeat of "Bad Kiss".
In "Quips", Skips argues that no one shooed away Don when he came to the park in "Don", and Rigby reminds him that was because everyone liked Don, and Rigby tried to get rid of him, which is true.
Continuity Porn: "Exit 9B" has every deceased villain and nearly every ally Mordecai and Rigby have ever encountered together for one big brawl out, and the Big Bad of the episode is the son of GBF. There's also the flashbacks the rest of the park workers have when they regain their memories, as well as the things Mordecai and Rigby do to snap Benson out of it.
Cruel Twist Ending: At the end of Muscle Man's story in "Terror Tales of the Park III " where the pumpkin head scarecrow turns Mordecai, Rigby, Muscle Man, and High-Five Ghost into pumpkins and smashed them for smashing his wife's head. The scarecrow used their seeds to grow them into pumpkins (just their heads) where they would try to convince any people who would want to buy them to buy the others instead. Muscle Man lampshades the ending when none of his co-workers were impressed by his story.
Dangerous Forbidden Technique: The Shredder from "Power Tower", a bodybuilding pose that can shred the competition if done correctly, but shred the poser to death if it's not.
A Date with Rosie Palms: In "Muscle Woman", Mordecai tells that a woman is single when she buys only one banana, leaving the viewer to infer the things she does with that banana.
In "Over the Top", Death's left arm is way bigger than his right arm. You later see him reaching into his pants... with his left arm.
It appears as early as "Caffeinated Concert Tickets" where Margaret's date references this, saying that Mordecai should "stop pumpin' it in the wind."
Darker and Edgier: For a christmas special in a kids-show, the Christmas Special turned out to be this. I mean usually presents disappear and people are sad or something along those lines. Quillgangets his hands on the present he created and wishes for the destruction of Christmas forever, which then causes BOMBS to be placed on anything Christmas-ish including snow. Thankfully we never see how powerful the bombs were but it's very likely they could do serious damage and had he succeeded it would've been the largest terror attack in history. This would've likely caused no one to want to celebrate a holiday that goes hand in hand for a global scale bombing.
"Fuzzy Dice" as well. The leader of the enemy gang is seconds away from shooting Rigby execution-style when the gang gets Killed Off for Real in a shoot-out with the FBI (tempered only by the fact that they're robots).
Deadly Prank: A near-deadly prank in "Prankless", in which Pops gets crushed by a bed that Muscle Man had taped to the ceiling. He gets taken to the hospital, where he just barely survives.
"It's supposed to say Rigby but they misspelled my name wrong."
There's also the 'Death Punch of Death' in "Death Punchies".
Rigby rents a movie from "Movie Shack Hut".
From "This is My Jam", the song Rigby gets stuck in his head is called "Summertime Lovin', Lovin' in the Summer (Time)".
"You jerks are jerks! You don't know anything about comedy!"- Muscle Man bombing at open mic night.
In "But I Have A Receipt", Mordecai and Rigby attempt to get a refund from "Comics Plus And More Etc."
The elf's name in "The Best VHS in the World" is ButtonWillow McButtonWillow.
Depending on the Writer: Either Benson or Mordecai and Rigby's statuses as the Designated Hero/Villain. In some episodes, Benson over-reacts, while sometimes, it's justified. Sometimes, Mordecai and Rigby screw around, but would try to fix what they broke.
Mordecai's competence. In some episodes, he gets dragged into slacking off by Rigby, but would try to suggest getting back to work. Other times, he willingly joins in on the slacking.
Rigby's competence and level of Jerkass also have some wild variations throughout the episodes.
Muscle Man hosts a fake party, gets Mordecai and Rigby turned into ghosts, and fakes a heart attack after Rigby accidentally spilled soda on him. After almost choking to death.
"Over the Top". Skips isn't too pleased about Rigby beating him in arm wrestling, especially after finding out he used an arm brace. So he forces him into a rematch, yanks out the brace, then instead of leaving it there, smashes him through the table, killing him. Skips realizes this though, and puts his own soul on the line to bring him back to life.
The Wizard from the Halloween Special turns Rigby into a house and brutally murders the entire cast just because Rigby egged his house.
Not to mention egging the house Rigby rather harmlessly, giving Rigby enough time to lampshade the topic... JUST before the wizard drops a house-sized EGG and drenching him in egg yolk as he screams in horror. "The End."
Doppelgänger Spin: Used by Chong in "Stick Hockey" as one of several methods to gain the upper hand in the final round of stick hockey.
Downer Ending: In "Steak Me Amadeus", Mordecai finally has the courage to ask Margaret to be his girlfriend, but she reveals to him that she's been accepted into her dream college so she can't be his girlfriend which devastates both of them. The last scene shows Mordecai sitting on the park roof with Rigby trying to comfort him.
Dragged Off to Hell: "Exit 9B" ends with all the surviving resurrected villains being sucked back into the underworld when Garrett Bobby Ferguson Jr.'s plan is foiled.
Dramatic Gun Cock: The animatronic bear reaches for his gun to kill Rigby before the FBI shows up in "Fuzzy Dice". The snipers cock their gun and the bear follows suit. Cue in a fatal shootout.
Dramatic Irony: In "Cool Bikes" Benson admits that Mordecai and Rigby are the coolest guys he knows. In any other context this would be sweet, but the viewer knows that Benson just unknowingly secured them a guilty verdict.
Driven to Suicide: Cart in "Out of Commission", to the point where he actually wants to be driven off a cliff to drown in a lake. While having fireworks exploding on him. After his death, Mordecai and Rigby have to bring him back due to Benson's math mistake.
Drunken Montage: The beginning of "Mordecai and the Rigbys" has Mordecai and Rigby drunk trying to find a good name for their fake band.
Drunk on Milk: Every time there's a bunch of soda cans around, they mean beer.
Dumbass Has a Point: Pops, while rather naive and obstructive during "Think Positive", does bring up the obvious question of why Benson has to constantly yell at Mordecai and Rigby. Sure, they're slackers, but it often doesn't make them much better employees.
Wedgie Ninja's sacrifice in "Go Viral" to save Mordecai, Rigby, and Pops from the Warden of the Internet, destroying her in the process.
Ear Worm: In-Universe example: in "This Is My Jam" Rigby gets "Summertime Loving, Loving in the Summer (Time)" stuck in his head, to the point that the song sings itself whenever he opens his mouth. Not singing it, but playing on its own. Eventually the song manifests itself into a physical form and starts driving everyone nuts. The gang decides to come up with an even catchier song to rid of it... "Aw snap! Aw snap! Come to our macaroni party then we'll take a nap!"
Which then gets stuck in Rigby's head just like the last song, causing everyone to groan.
When you tie in rock paper scissors 100 times, it summons a black hole that apparently leads to a monster's dimension. The glimpse of the monster is…disconcerting.
In the name of all that is holy, do not play with the wires of the Destroyer of Worlds arcade game.
Father Time from "It's Time" is one. He is a being made entirely of clocks who lives at the end of time, and was actually a rather nice guy and was willing to let Mordecai off with a warning and press the Reset Button for him.
The Snow Monster. He's made of snow, breathes fire from a jawless mouth, and creates one of the scariest moments of the show.
Actually, the "Book of Park Records" could qualify too, as it gave a Necronomicon vibe when they touched it, and can alter reality (at least within the park).
Peeps from the self-titled episode. He's a giant floating eyeball that can summon a huge number of smaller eyeballs from his body and threatened to watch everyone till they died. When they made a deal to get rid of him, he wanted all their eyes if he won.
SUSAN. The worst part is how normal she seems before turning her head 180 degrees and going One-Winged Angel.
The mirror monster in "Jinx" could qualify too.
Eldritch Location: The whole world turns into one if you eat too much sugar. The laws of physics, time, sound, light, and colors start to break down, leaving almost nothing as it was.
The eye creature in "Peeps" grows several extra eyeballs, which Rigby then scorches with a laser pointer.
The last of which BURSTS INTO FLAME.
Also, the bet Peeps makes where if he wins, he get everyone's eyes. At which point, he produces an ice pick and ice cream scoop.
In a subtler example from the same episode, Mordecai loses the ability to blink and his eyes become so irritated that the eyedrops he uses evaporate before even touching them.
Face Doodling: Rigby does this to Pops when he passes out in "Dizzy".
It's also the main thing the Creepy Doll in the Halloween special does.
Faceless Masses: Averted, in almost every crowd shot the background characters are drawn about as detailed as full on screen ones, although they are often duplicated, mirrored and huge crowds are usually not animated.
Fail O'Suckyname: In "Trash Boat", Rigby decides to change his name to two random words after seeing a news report on a rock star who did the same thing. The name he comes up with, Trash Boat, is... less than inspiring, and he spends the rest of the episode trying to get it changed back.
Fake Brit: Future Mordecai and Future Rigby, in "Mordecai and the Rigbys". They lose the accents right before the two of them are destroyed by Present Mordecai.
Even worse in the Christmas Special. We actually see the very human Santa getting shot numerous times. Lucky he was wearing a Bulletproof Vest.
Fawlty Towers Plot: Most episodes start off with a problem that's bound to escalate into supernatural chaos.
Felony Misdemeanor: Muscle Man rams a table into the wall and rolls on the floor screaming when Rigby accidentally splashes soda in his face.
Skips quits the park bowling team and leaves his job altogether because he didn't want his friends to know that his name used to be "Walks" until he changed it due to the fact that he was tired of being asked "Why are you called Walks when you always skip?"
You can be sentenced to death for being too cool.
In "Butt Dial", exceeding your log-in attempts can get you erased from existance. It turns into An Aesop about phone hacking (which actually is heinous) though.
First Kiss: "Bad Kiss" and "Do Or Diaper" are all about Mordecai trying to get it right with Margaret. They end up not kissing in either episode, though. "Picking Up Margaret", on the other hand ...
Fist Pump: Parodied in in the episode "Caffeinated Concert Tickets". Mordecai and Rigby want to get concert tickets for a band of the same name. They do this pose many times throughout the episode while shouting "Fiiist Puuummmp".
Five Stages of Grief: Muscle Man in "Trucker Hall of Fame", after his father was killed. First, his brother John walks into his trailer teary-eyed, and Muscle Man doesn't believe anything. At the funeral, he breaks out in tears. While in the forest, Muscle Man takes his picture of his dad wearing a trucker hat and smashes it. Finally, after Muscle Dad's ghost appeared, Muscle Man spread his dad's ashes in the Trucker Hall of Fame.
Five-Temperament Ensemble: Benson (choleric), Skips (melancholic), Mordecai and High Five Ghost (leuquine), Rigby and Muscle Man (sanguine), and Pops (phlegmatic).
Flanderization: Inverted. The show actually downplayed several characters'(e.g. Muscle Man, Benson, Skips) most defining traits.
Foreshadowing: Throughout the show, Margaret has been trying to transfer to her dream college. In "Steak Me Amadeus", Margaret calls Mordecai and tells him that something came up and she'll be late going to the restaurant. From that moment you can tell that the date won't end happily when they finally meet up at the restaurant and Margaret has something important to say to Mordecai.
Gila monster, pigeon, marmot, dik-dik, baboon, bushpig, long pig, aardwolf, manatee, bat-eared fox, red bat, spotted bat, dog-faced bat, opossum, emu, hairy long-nosed armadillo, big hairy armadillo, screaming hairy armadillo, colocolo, spectacled bear, lowland tapir, red deer, shy albatross, komondor dog, hagfish, sonoran sidewinder, snorkel viper, oak-leaf rattler, bacon, narwhal, kookaburra, aha ha, blobfish, bulldog stargazer, tufted titmouse, screaming cowbird, green jay, lady amhersts pheasant, polish chicken, quetzal, surinam toad, tomato frog, turtle frog, Christmas tree worm, vampire squid, coelacanth, mugger crocodile, aya-aye, dumbo octopus, platypus... and more!
Friend Versus Lover: It would be a bit disingenuous to call them lovers (definitely at that point in the series, anyway), but Rigby often seemed to set a "her or me" dichotomy with Mordecai vis-a-vis Margaret. This trope helped set the conflict for "The Unicorns Have Got to Go," "It's Time," and helped inspire Rigby's jerkish behavior in "Do Me a Solid."
Friends with Benefits: Inverted in "Diary", after Mordecai says he and Margaret are friends, Rigby responds with "Yeah, your friend without benefits!"
Freudian Excuse: "Think Positive" implies that Benson is so quick to raise his voice because his father told him, "You'll never get anything you want in this world if you don't yell for it."
Frothy Mugs of Water: Basically invoked in "Over the Top"; when the gang head out for "drinks", Mordecai and Rigby anticipate, respectively, ordering a Watertini and an Ice on the Rocks.
Funny Background Event: In "First Day", the other want ads in the newspaper are largely humorous. "...and dancing ONLY. Must be over age 75, and no dyed hair or wigs allowed."
Furry Confusion: Subverted in "A Bunch of Baby Ducks." The titular ducks seem to be non-anthropomorphic at first, but quickly pick up humanlike behaviors (like karate chopping) from Rigby, and their mother speaks English.
Played straight in "Skunked," with Mordecai and Rigby picking up roadkill in the park and even PLAYING A BINGO GAME USING THE DEAD ANIMALS, though technically it was created by Benson to motivate them to work; they can't stop working until they get Bingo.
"The Mississippi Queen". A concoction so absurdly spicy that Mordecai, Rigby, and Benson spent the rest of the party they were attending experiencing a Disney Acid Sequence played to the tune of "Mississippi Queen" by 70's rockband Mountain. It even came with a little umbrella.
Mordecai has a flashback of doing this to Rigby in "Death Punchies."
Rigby's favorite internet video is of an ostrich kicking an old man in the balls.
Mordecai does this to GBF. But since said champ is basically a head with arms and legs, the champ reacts with "my chin!"
Subverted and played straight in "Rage Against the TV". In Mordecai and Rigby's video game, the duo are punching one of the games' bosses in the junk. As in the miscellaneous things he is carrying. Later, Skips gives one to The Hammer with a chair.
Skips perform one on a zombie. It doesn't work.
Half-Dressed Cartoon Animal: Rigby started walking around naked one day. His brother thought that was so cool, he hasn't worn clothes since.
Skips is a straighter example, wearing jeans but no shirt.
Benson: Ugh! When the game starts, you guys have to...
This also happens in "Skips Strikes".
Death: You! Get your balls! (chuckles)
Heroic Sacrifice: In "Benson Be Gone", Leon barrel-rolls Mr. Maelard's limousine and knocks off Susan's heels, sending Susan back to hell and the limo careening into a wall where it explodes, killing Leon.
In "Go Viral", Wedgie Ninja stays behind to stop the Warden of the Internet, allowing the other viral videos to go free. He keeps it up until she explodes, killing himself in the process.
In "Appreciation Day", Rigby manages to get the cover of the Book of Park Records back to Mordecai, but he gets killed for his trouble.
Donnie G in K.I.L.I.T Radio is a subversion. At first it looks like a Heroic Sacrifice, because he gets a hole blown in his stomach and is at ground zero of the explosion that wipes out the radio station...and then in the end of the episode, he's inexplicably alive and hanging around Mordecai and Rigby with no explanation.
Hideous Hangover Cure: After getting drunk and drugged from the Mississippi Queen in "Weekend at Benson's", Mordecai, Rigby and Benson wake up hungover and stoned, and Benson gets cured when he founds out his neighbor gave him her cell phone number. As for Mordecai and Rigby, they get cured when Benson rehires them.
Hipster: In "Cool Bikes" Mordecai and Rigby dress in this style to be cool. The stack of Brain Explosion records they swear upon in court is even a Call Back to Mordecai's own infamous hipster moment in "This Is My Jam".
The Movie Shack Hut employee has traces of this, preferring VHS to DVD.
Chad and Jeremy from the episode "Replaced" are true hipsters. They dress like hipsters, are obsessed with their phones, are so self-obsessed that they think they can fit their job around their schedule, and talk in a very arrogant way. It was clear that they are just as useless as Mordecai and Rigby, but didn't joke around, so Benson thought they were serious.
A rather... odd case in this one: in the episode "Karaoke Video", Benson tells Mordecai this:
Benson: I want to... you know, boost morale around here.
14 episodes later, in "Think Positive", we get this:
Pops: Stop yelling! It's bad for morale around here.
Benson: WHO GIVES ABOUT MORALE???
Hypothetical Fight Debate: Invoked and double subverted in the episode "Replaced". Mordecai and Rigby try to prevent their replacements from finishing their work by having them debate who would win in a fight between a two-headed dinosaur and a robotic Bigfoot. They both agreed that a robotic Bigfoot would win. Then Mordecai gets into a debate with them on why a two-headed dinosaur would win. It still didn't stop them from working.
I Call Her "Vera": Rigby lets Mordecai name a keyboard he stole. He calls it 'The Power'.
Mordecai: I always wanted to date a girl named The Power.
I'll Kill You!!: Rigby on occasion, with varying degrees of seriousness.
Mordecai says this in "It's Time" right before accidentally killing Rigby.
In "Death Punchies", Rigby actually tells this to Mordecai several times, and means it. He tries to kill his best friend with the Death Punch, and they end up trapped in a pit fifty feet underground with lava pouring out the walls.
In "Cool Bikes" Mordecai and Rigby are seen with two human girls in the cart, and in "Cruisin'" they tried to pick up several ones.
In "Yes Dude Yes" Rigby set up Mordecai with several woman, including three human, a flamingo and a canary, and he ends up with a humanoid cloud called CJ. Also when CJ's (human) friend's tought Mordecai was good-looking.
Margaret is seem with several human boyfriends in multiple episodes. In "Cruisin'" she along with Eileen pretend to have human boyfriends.
In "Muscle Woman" Mordecai (reluctantly) dates Starla in an attempt to get her back with Muscle Man.
Margaret's parents, a female Cardinal and a male Human.
Benson has a relationship of sort with an human called Audrey despite him being a gumball machine.
Although they are both birds, Mordecai and Maregaret are of different species and could be seen as an example.
Eileen crush on Rigby. And in the episode "It's Time" Rigby asked Margaret out on a date (the writer implied that Rigby was going out with Margaret pretty much just to get under Mordecai's skin, but the point still stands).
"I can fix this" from the hot dogs episode. First, it's Rigby trying to fix everything that he's messed up but only messing it up worse. Then it's Rigby trying to keep Mordecai from dying. Then when Rigby brings up who bought the premium hot dogs...
There's also "I wouldn't want to ruin the surprise" from Future Mordecai in the episode "Mordecai and the Rigbys" when the regular Mordecai asks if he ends up with Margaret in the future. Mordecai throws it back at him in his Shut Up, Hannibal! speech when he breaks up the band.
"Don't blink!" in "Peeps". First said by Peeps to Mordecai when Peeps starts growing extra eyes to tease Mordecai into blinking, and then by Mordecai to Peeps when it finally looks like Peeps is going to give in and blink.
Is This a Joke?: Lollipops as currency. Also, "he can't make a major speech facing away from the audience!" From the same episode, Mordecai draws Pops' face on the back of his head because Rigby vandalized his face with Sharpie drawings.
Jerkass: Benson, by far and Muscle Man. Rigby and Mordecai can act like this a lot to each other, if they get on each other's nerves too much.
In "The Best Burger in the World", Benson makes Mordecai and Rigby work on all their extra jobs that they slacked off on, despite there being a time limit to get said burgers. So he gives them the most work, and honestly expects them to not try to get burgers. Finally at the end of the episode, he eats both which they bought as punishment. Uh Benson... you could have had them gotten the burgers before and then have them do the work. Instead of being a jerkass...
Karmic Transformation: Egg a wizard's house, be cursed to turn INTO a house. Poor Rigby learned this the hard way in the Halloween special.
Keep Circulating the Tapes: In-Universe; Mordecai, Rigby, Muscle Man, and High Five Ghost find an old Laser Disc inside some boxes. In their search to find a laser disc player, they learn that every laser disc and player in existence had been destroyed by VHS in a literal format war that happened 2 years prior. All except for their disc, and one player, and recovering that last player unravels a big conspiracy.
Averted in Real Life, as Regular Show's first two seasons are available on Blu-Ray and DVD and Netflix has season one uploaded (though, if you hate it when networks go back and edit reruns, then the best bet is the Blu-Ray or DVD version, as the Netflix version edits Rigby's line "How in the H are we gonna fix this S?" to "How in the heck are we gonna fix this stuff?" and replaces all mentions of "pissed" to "ticked").
Kick the Dog: Benson, throughout "The Best Burger in the World". He knows there's a time limit for Mordecai and Rigby, so he chooses then to force them to do a catch-up rush. And at the end? He outright steals the burgers that Mordecai and Rigby legally purchased.
Kill 'em All: According to Benson, the only way to get rid of unicorns.
The Destroyer of Worlds too.
Skips: You fools! Destroyer of Worlds will kill us all!
The final story in the Halloween Special has this happen with the entire cast.
Knight of Cerebus: Garret Bobby Ferguson Junior, who makes the fourth season premiere play out much more seriously than other episodes.
Knight Templar: The Warden of the Internet. Ok, wanting the internet to be used responsibly, that's an alright motive. Confining anyone who makes a viral video to a Fate Worse Than Death? Yeah, that's insane...
Konami Code: Used to dodge bottles being thrown at Rigby's character in a Rock Band style video game.
Lampshade Hanging: The episode "High Score" is practically made of this. note Margaret: "I can't believe we were so fascinated by a video game, but we were!"
At the beginning of "Butt Dial", Mordecai and Rigby mention how their 'game night' with Eileen and Margret was successful, without anything supernatural or park destroying happening.
Line-of-Sight Name: In "Trash Boat", Rigby decides to change his name to two completely random words. He looks over at the overflowing trash can, then at the painting of a sailboat on the wall, and hence the title of the episode.
There's a Hard Work Montage to the tune of Loverboy's "Working For The Weekend" while trying to save up enough money to go to a concert.
In "Death Punchies", Rigby learns to Death Punch to "You're the Best" by Joe Esposito.
There's also Mordecai's, Rigby's, and Benson's slack off sequence, all done to the tune of an instrumental song based off of Boston's "More than a Feeling".
Mordecai and Rigby end up starting a bar fight over possession of a karaoke tape with the owners of the bar while Pops sings "Footloose" by Kenny Loggins.
When Mordecai, Rigby, and Benson attend a party, they drink a spicy concoction called the "Mississippi Queen", named after the song by the rockband "Mountain". They spend the rest of the party in a Mushroom Samba while the song plays.
In "Eggscellent", Mordecai trains to eat the Eggscellent challenge to "Holding Out for a Hero" by Bonnie Tyler.
In "TGI Tuesday", Mordecai, Eileen, and Rigby get into a dance off against three party ghosts from 1985 to "Ballroom Blitz" by The Sweet.
Mr. Exposition: Skips, when he gets involved in the current trouble of the episode, usually knows exactly what's going on and explains it further and how to solve it. Being a few thousand years old may have something to do with this.