One early episode features husband & wife arms dealers coming in straight from a gun show where he got shot in the leg. The wife shows off the merchandise... That Came Out Wrong. The wife tries to sell guns in the reception area, letting people handle non-functional samples (legally they're not allowed to sell functional grenade launchers but they sell 'kits' that make them functional) . Jerry the Receptionist picks up a grenade launcher and speaks of using it on the neighbor's dog who keeps doing his business in Jerry's yard. He puts the grenade launcher to his shoulder and mimes tracking the dog. "Woof...Woof...Woof.." Then he pulls the trigger and fires a grenade into the parking lot, where it blows up an ambulance.
Wife: Damn...I guess Earl used a kit on that one.
In the episode called "Homecoming", Dr. Corday had been having a bad rash all over her hands after a trip to the woods with Dr. Greene. When Dr. Romano asked about her getting exposed to poison ivy, she just looked back at him blankly(being British, she had no idea what it looked like). She asks if it will spread and Benton says that it will spread to other body parts that it has contact with. Elizabeth mutters, "Oh oh", and the next shot is Dr. Green scratching his crotch. But the joke just keeps on giving, Malucci sees him scratching in front of the new med studants and asks him if there's a "problem with the australian boys? You keep scratching down under". Then while Elizabeth is giving him some cream and a shot for the rash, Haleh enters and can barely hold back laughter as she catches them.
A season 7-ish episode involves prankster Dr. Malucci having been hit with a sedative after a prank gone wrong. He remained pretty much passed out the rest of the episode. Luka decides to have a little more fun with him by dermabonding his hand to his forehead.
In a season two episode, Weaver has set up a camera in a trauma room in order to critique the way personnel handles cases. As she's reviewing the tape for the ER staff, she rewinds it to find an accidentally recorded conversation between Mark and Doug. Earlier in the day, Mark had pulled Doug into the room to confide in him that he was nervous about his upcoming date, the first since his divorce. Thanks to Kerry's fumbling, the entire ER staff gets to hear him confess to Doug that the only person he's ever had sex with is his ex-wife.
"Fathers and Sons": Doug has just told Mark that he's reconciled with Carol. Mark is stunned to hear this (Doug had cheated on her left and right the first time they dated).
Mark: Does she make you take a lie detector test every week or something?
In an early first season episode, a patient comes in with heroin withdrawal and keeps grunting the same note over and over, sounding like a car alarm, while Greene and Lewis work on their write-ups. Hilarity ensues.
To say nothing of the "Peter, I'm pregnant" montage that ensues when Benton is waking up afterward.
Carter is passed out grabbing a nap in an empty exam room. Greene and Lewis, being two of the more mature members of the staff, busy themselves with putting his leg in a cast. Then they have the receptionist page him, leading to Carter stumbling groggily into the hallway before losing his balance and falling on his face.
Later in the same episode, a scary-looking biker who had brought his wife into the ER proceeds to grab a saw and cut the cast off for Carter, much to Carter's dismay.
Carter is bringing a patient in while Benton grills him on various medical details of the patient before asking, in the same demanding tone of voice, for the patient's mother's maiden name. Carter frantically flips back and forth through the patient's chart before nurse Haleh informs him that the last bit was a joke.
A Running Gag in the otherwise serious episode "Random Acts" is when the nurses and doctors find a trashy novel with characters based on everyone in the show, and they decide Carol wrote it.
Abby giving her version of the ER tour to the new residents.
In a late Season 12 episode, a deaf guy and his girlfriend come into the ER after he got beat up by the cops and she OD'd on drugs. Luckily Ray and Jerry are able to sort the situation out before long. The cops come around to apologise with Jerry translating. The boy signs back "I have a special sign for you" and then proceeds to flip off the cops. Jerry sheepishly tells the guys that he doesn't need a translation for that one.
The DVD Outtakes.
George Clooney appeared to have a tendency to prank his fellow colleagues while on set. Two incidents stand out:
The first involves Jerry pulling out a twinkie and delivering his line correctly, then afterwards going "it's a little lubey." (He'd covered the prop with lubricant)
The second time he proceeded to scare the crap out of Julianna Marguiles by rigging a drawer.
One memorable outtake has Paul Mc Crane (in a scene where Lucy bangs on Romano's door in the middle of the night to find him very pissed) somehow hide another (gigantic) actor, wearing nothing but a towel and carrying a pair of handcuffs and a bottle of wine, behind him. Mc Crane then proceeds to flirt with him, shoo him upstairs, and call him "honey" before asking Lucy to join them. Kellie Martin, the director, and the tech crew utterly lose it.
Alex Kingston proceeds to start kissing Laura Innes out of absolutely nowhere. Anthony Edwards proceeds to react by turning to the camera, giving a grin and a thumbs up.
Scott Grimes delivering this line off-camera: "This marriage is clearly a marriage truly made in the ER, because if it was [Mercy or Northwestern originally] House or Grey's Anatomy there'd be better food and booze!" Everyone loses it.
Another attempt: "I'd just like to say...just like to say...that this is clearly not working." Maura cannot stop corpsing.
One scene has Dubenko explaining to Neela what a key party is. She walks off screen and clearly states "Makes my labia wet" in one of the most cheery monotones possible. Leland and the extra in that scene proceed to go off-camera and crack up.
John Stamos and Goran run through a scene correctly together, only for Goran to pause and state "I thought I fucked up but I didn't."
The cops are proceeding to walk Pratt outside and then Kerry jumps after them, attacking the guys begging them not to take her, only to utterly screw up that scene (I believe this was Laura Innes final episode)
"Tell them I'm pregnant, maybe they'll cut me some [slack originally] snack and...(trails off into laughter)" Doubly funny as Sherry Springfield really WAS pregnant at the time.
Sherry delivering a line to a patient, ending with "but in situations like these...". Someone off-camera yells "The memory goes first!" All she can do is swear.
La Salle's inability to pronounce "Megalomaniacal." (this apparently turned into "Maniacal sadist" in the end)
Paul Mc Crane absolutely ruining a take by finishing the scene perfectly, only to then turn to the camera and pull a face that is completely inappropriate to the scene.
A rare non-ER example: during one of the Season 9 outtakes, the monkey proceeds to escape out of the net shot at it.
Ming-na drops a lid off of a medicine bottle. She bends to pick it up and one of the crew yells out "Not going to this hospital, I'm telling you that." Everyone cracks up laughing.
Mekhi and Sharif proceeding to play-wrestle mid-scene.
Anthony Edwards encouraging Alex Kingston to moon the camera while singing the Hokey Pokey.