Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.

The Mission Brief

After giving Reisman the basics of the mission, Gen Worden asks him his thoughts. There is then a quick cut to Armbruster, a personal friend of Reisman's, giving his head a tiny shake, trying to get Reisman to not give his honest opinion. Reisman bluntly gives it, that whoever thought up the operation had to be a certifiable nutjob, Worden angrily reprimands him. Once Reisman and Armbuster leave, Worden chuckles and admits to General Denton that Reisman probably isn't wrong.

The Prison

Arriving at Parachute Training

As Reisman and his men arrive to fanfare:
Bravos: Sounds like we're running into a party.
Reisman: Boy, you don't suppose it's for us, do you?
Major Reisman ad-libbing after Colonel Breed requests an inspection of his troops:
Reisman: "Who wants to be a general? ... Pinkley?"
Pinkley: "What kind of general, sir?"
Reisman: "Just a plain, ordinary, every day, home-lovin' American general."
Pinkley: "I'd rather be a civilian, sir."
During the inspection, Pinkley warmly asks a soldier where he’s from, the soldier enthusiastically says “Madison City, Missouri sir!” Pinkley drops his smiles and deadpans “Never heard of it.” The soldier’s look of instant disappointment is hilarious.
The scene is punctuated throughout by the band conductor’s obliviously stupid grin that never changes even as Colonel Breed commands the band cease. The Conductor misses the cue and continues playing with his big, goofy grin unaware that Breed is obvious to Reisman's ruse.
When the men move in and Breed calls Reisman out on Pinkley's stunt, he doesn't even flinch.
"I owe you an apology. I always thought you were a cold, unimaginative tight-lipped officer. But you're really... quite emotional aren't you?"

Breed raids the Camp

When Col. Breed bullies his way into the camp and Posey recognizes the two guys who beat up Wladislaw, the Dozen pass it on to each other, until it's Maggot's turn:
Maggot: (turning to the guy next to him) Those are the two guys that beat up Wladis- (realizes it's Wladislaw, then looks away awkwardly)
- Charles Bronson's stern face is the perfect punchline.
The glee at which the Dirty Dozen - sans Maggot, who just stands there - disarms Col. Breed's men when Reisman shows up to stop Breed.

War Games

When Jefferson's group steals the ambulance in order to ambush Breed's HQ the doctors notice he's wearing the same insignia they are. His response? "That's right, we're traitors."
General Worden trying to keep a straight face during the War Games when he realises the 'wounded soldier' (Posey) is stealing gun parts and the whole thing's a setup. He pulls himself together in time to bail before the Trojan ambulance arrives.
Bravos pushing Major Armbruster out the ambulance prior to them taking over the HQ; Armbruster was a good sport about it, limping to the HQ and laughing along with the Dozen holding Col. Breed and his men at gunpoint.

The Mission Itself

In the final briefing before the mission, Major Reisman describes the assault phase as "kill every officer in sight." One of the Dozen quips, "Ours or theirs?" Despite being the team's only officer, the Major rolls with it, adding, "Well let's start out with theirs..."
EVERYBODY ABOVE THE RANK OF LIEUTENANT TO THE BOMB SHELTER! IMMEDIATELY!
During the bulletfest, the remaining members attempt to find a vehicle for a getaway. Franko finds a transporter and the following non-verbal exchange happens:
Franko "I've found one!" (enthusiastic thumbs up)
He starts the transporter, then notices it's attached to a light machine gun.
Reismann "Really??"
Franko "Oops."
- Franko looks for a vehicle:
  Can you believe these guys? They keep these things locked!

Behind the scenes

While filming in England, Lee Marvin was in a nightclub and propositioned an elderly woman in the most vulgar manner possible...who just happened to be Sean Connery's aunt. Because Marvin slurred his speech, she politely asked him to repeat his proposition, and just as he was doing so, Connery was on his way over to thump him. He was saved from a beating by the film's producer, who told Connery not to hit Marvin in the face...because he had closeups the next day. Connery, appreciating the joke, laughed and said, "You fucking producer", before walking off.

Funny / The Dirty Dozen