Defictionalization: Several things, but perhaps most prominently the "Silly Party" of the Election Night Sketch inspired the Real Life Monster Raving Loony Party. The Overly Long Name of a candidate was also borrowed by a real British election candidate.
In the "Mouse Problem" sketch from Episode 2, the address and phone number of an interview subject who has admitted to wanting to be a mouse are displayed and read out by John Cleese. In the original broadcast from 1969, the phone number was David Frost's home phone number, and after fielding a large number of prank calls, an annoyed Frost complained to the BBC, who edited the number out of the first re-runs in August 1970.
In the first run (1973), show no. 38 started with a Party Political Broadcast that was choreographed. Upon the syndication rights changing in 1983, the brainiacs at Time-Life (first syndicators) erased it. It was replaced with brief superimposed titles. The first DVD release of the Monty Python 16-Ton Megaset delivered an even worse blow: the previews of BBC comedies ("Dad's Pooves", "Up The Palace", etc.) at the end were left out.
Executive Meddling: The BBC frequently got cold feet over some of the Pythons' humour, resulting in often awkward acts of censorship.
If the episode with the Tudor porn shop seems disjointed, that's because it had to be edited rather severely before it was allowed to air.
The "Political Choreographers" sketch was edited out of one episode after its initial broadcast and apparently only survives in a low-quality off-air recording.
Additionally, most episodes of the two Python precursor series, Do Not Adjust Your Set and At Last the 1948 Show, have been wiped. This could have happened to Flying Circus itself were it not for the actions of Terry Gilliam, who bought the masters as soon as he could.
There is an animated section in between the "Crackpot Religions" sketch and "How Not to Be Seen" involving Jesus and the two thieves being crucified on telephone poles, while an Alter Kocker Satan (played by Idle) appears out of the ground. It was cut after its initial showing, can only be seen in low-grade, black-and-white footage, but the image of the crucifixes can be found in a split second when the episode is "recapped" in fast forward later in the show. Now available in full color.
In the narration to one Gilliam animation they crudely replaced the word "cancer" with "gangrene".
In the "Summarize Proust Competition" they muted Chapman's reference to "masturbating". As one of the Pythons put it, he was allowed to mention killing animals, but masturbation? That's just horrible.
They would only allow the "Undertaker Sketch" to be recorded if some of the studio audience were seen protesting about it. Even so, the sketch was apparently cut from the master tape after transmission, and had to be reinserted from an NTSC copy.
The "Quiz Show"/"Spot the Brain Cell" sketch was cut from BBC repeats for several years, but restored for DVD release.
Some of the BBC's complaints stretched into downright paranoia. In the "New Brain from Curry's" sketch, a representative for the brain's delivery asks a Pepperpot to sign a fake leg; when the Pythons submitted the sketch for review, they were told to "cut the penis." The angle that Cleese held the leg into the doorway caused it to resemble an oversized Johnson.
The infamous "Wee-wee" sketch is one of only two filmed sketchesnote The other, one about an artist that makes a statue of Cleese only to give it an incredibly long nose, was cut for unknown reasons that have not only never aired but also failed to surface from the vaults. It revolved around a wine connoisseur being served urine by a French waiter and repeatedly believing he's drinking fine wine ("No, sir, zat is wee-wee."). The BBC didn't like it because one of the wine glasses was slightly rosť (pink), which they took to mean menstrual urine. Eric Idle protested, but the excuse was good enough for John Cleese who detested this sort of humor and managed to get the sketch canned for good.
Even worse was the show's treatment by ABC in the US, which involved cutting shows for timing and removing anything the network considered objectionable - which, the Pythons claimed, included almost everything that was funny. This led to a full-blown lawsuit between the Pythons and ABC, the ultimate result of which was the full reversion of copyrights to the Pythons' own company.
Fake American: Many cases, to varying success. Even Terry Gilliam, who is American, had trouble sounding like it. For specific examples...
In the Marriage Counselor sketch, John Cleese plays a random cowboy with a laughably thick accent to help give a pep talk.
Bicycle Repairman has everyone attempting American accents, though a dead giveaway is when Cleese uses the word spanner instead of wrench.
John Cleese's narration in the Science Fiction Sketch goes an over-the-top Hollywood-style narration.
The Twentieth Century Vole sketch casts the entire group as Hollywood film execs.
The Attila The Hun show is done in typical American 50s sitcom fashion.
Hey, It's That Guy!: Relatively few roles were played by anyone outside of the Pythons themselves, and were normally relegated to whenever they needed more than six men on screen at a time, and Carol Cleveland normally played the more buxom women.
However, the episode "Mr & Mrs Brian Norris' Ford Popular" closes with a fake talk show hosted by the "It's" Man. His guests are British singer Lulu and Ringo Starr. As themselves.
Also appearing as himself, Richard Baker, a well-known straight-laced news anchor of the BBC, who's rather confused about lemon curry.
Thames TV continuity announcer David Hamilton appeared as himself in Episode 39, which opens with the Thames TV logo and Hamilton declaring, "We've got an action-packed evening for you tonight on Thames. But right now, here's a rotten old BBC programme!"
In addition to collaborating as a writer on a few sketches, Douglas Adams appears briefly in at least two sketches, once as a surgeon. Incidentally, this appearance was on Episode 42.
The Other Darrin: A couple of times one of the actors was needed to play another character, and was replaced mid-sketch once their lines ran out:
In "Court Charades", the jury foreman (Palin) and the defendant (Jones) were also two members of the Spanish Inquisition. When the defendant says "I wasn't expecting the Spanish Inquisition", the scene cuts to film of the Inquisition racing to the court house, and then cuts back to the studio when the Spanish Inquisition storms in; by which time the defendant has been replaced, and the foreman seems to have disappeared altogether.
In the "Father-In-Law" sketch, the father is played by Graham Chapman; when the sketch comes back as a link, he is replaced by Terry Gilliam.
On live stage productions, Eric Idle would sing the Lumberjack Song instead of Michael Palin.
Throw It In: During a sketch with John Cleese where they played a pair of Pepperpots, Graham Chapman suddenly went off script and screamed "BURMA!" for no reason. Cleese then asked Chapman, in character, why he did that, to which Chapman replied, in character, "I panicked!". It was later decided it was too funny not to include.