Anachronic Order: "The Passing", which takes place between "Dead Center" and "Dark Carnival", is played after both. "Swamp Fever" was skipped in favor of "Hard Rain" for the fourth season, and the fifth goes way back to "No Mercy" in the first game.
The Cameo: Archaon shows up in The Port around Bill's body while the rest are screaming. Only problem is, Bro and Mastermind are still there. This leads to even more screaming.
Aovnick (and Ramirez, apparently) makes a guest appearance in Season 5 as the helicopter pilot, first announcing that he can't pick the others up in the street because Puppy is too fat (suggesting they head to the hospital to get him liposuction), and then expressing disbelief in how many times they all die before he can arrive at the roof.
Camping a Crapper: David, Bro, and Mastermind are all checking out some graffiti in a bathroom in The Passing when a Charger comes in and barrels through all three of them.
Noodle Incident: In Episode 16 of Season 4, Mastermind keeps trying to talk about this one time where PuppySky's car broke down. However, whenever he does, PuppySky immediately screams at the top of his lungs to censor anything Mastermind says.
N-Word Privileges: "Does this mean I can say the N word without getting in trouble?""NOOOOO"
Obfuscating Stupidity: Debatable, but it seems that no one in the cast is actually a total newbie to the game, given Djy1991's background and that no achievements are earned in the first two seasons.
Oh, Crap: Djy1991's reaction after being greeted by a Hunter.
(Bro is puked on by a Boomer) Mastermind: Hey Bro. You know what that is on you? That's the Source engine. (Car alarm goes off) Aovnick and Djy1991:Oh shit/Oh crap!!!
Reactive Continuous Scream: Djy discovers Bill's dead body in The Passing and starts screaming. Mastermind then belts out his catchprase, Bro walks in and begins screaming as well. Aovnick stays silent.
Happens a lot, in fact. Another example is in episode 10, when a Charger knocks over Djy out of nowhere (around 3:52).
Running Gag: No matter where PuppySky is, a Charger will find him.
Not to mention Djy1991 and the Hunter.
Almost every time Ramirez' name is said in season 2, Sergeant Foley's head is overlaid on the screen.
One teammate keeps shutting the safehouse door at the end, preventing Djy1991 from getting inside. Another teammate then incaps the first , allowing Djy to enter.
Djy1991(playing as Rochelle): Let me in! Archaon: No girls allowed.
Finally subverted when Djy is the only one who manages to make it into the safe room alive in episode 5, wherein Nick tells him that girls ARE allowed in this one room.
Taking You with Me: In the first episode, the Charger manages to do this to Djy1991, crashing into him while at the edge of the building, meaning that both the Charger and Djy1991 died. It nearly took out Aovnick as well, though he managed to grab the ledge.
Team Pet: Ramirez in season 2, to the point where not even PuppySky seriously considers leaving him behind. He doesn't physically show up again afterwards, but he is referenced quite often.
What the Hell, Hero?: Archaon burns his teammates to death in the second episode, because he spent most of the first one dead.
Word Salad: Archaon engages in one when the team takes the elevator in the mall.
Archaon: I have high respect for this game if you can drive a car...and, auerh...ifyoucandriveacaryoucangojustaboutanywhereyoulikeandyouknow, youcangoanywhereinlifeandbeaverysuccessfulmanandallthatfreshairandexercisewilldoyoualotofgood. 'K, let's go. Aovnick: What the heck did you just say?