Blooper: Given the insistence by Schulz and Mendelson on using actual children to play the Peanuts characters (and child-actors weren't really a thing in 1965), it's no surprise some lines were flubbed.
Lucy calls the fear of cats "ailurophasia" instead of "ailurophobia".
Sally trips over the line "All I want is what I have coming to me. All I want is my fair share." This is due to the fact that the girl who voiced her was so young that she hadn't learned how to read yet and had to be fed her lines word by word, and in some cases, syllable by syllable.
Channel Hop: After airing on CBS for 36 straight years, the special moved to its present network home of ABC in 2001.
Christmas Rushed: Schulz and his team were given six months to complete the special in time for the holiday season, and animating didn't start until the third month in.* Most of the first two months was spent trying to figure out how to properly animate such artistically abstract characters - translating a comic strip like this to animation had never been done before (aside from Melendez's own Peanuts commercial spots). The result speaks for itself.
Dawson Casting: invoked Famously averted. Schulz insisted on having actual children voice his characters, the first time something like that had ever been done in a cartoon. However, some of the kids in this special were so young they couldn't read well yet (or, in Sally's case, at all), so they had to be fed their lines, leading to the stilted delivery that became emblematic of the franchise.
Defictionalization: Replicas of the little flimsy tree Charlie Brown gets are now being sold.
Edited for Syndication: In its original 1965 broadcast, this cartoon clocked in at just over 26 minutes. Nowadays, American TV networks have a lot more time devoted to commercials than back then. So for most of its subsequent airings, several scenes have been cut short (the first dancing scene) or cut out entirely (much of the beginning). Even in its first VHS release, the scene of the kids throwing snowballs at the can on the fence was cut out. It was restored for its second VHS release and was re-instated in network broadcasts in 1997 (with the Coke can redesigned).
Coca-Cola was the special's original sponsor, and a brief scene mentioning this was animated for the broadcast. Once their sponsorship ended, that scene had to be excised and was thought lost for decades until a film reel containing it was discovered, digitized, and uploaded onto the internet (rights issues prevent it from being on the DVD cut, though every other scene cut over the decades is restored).
Interestingly enough, Coca-Cola offered to restore the cut sponsor tags when the special was remastered, but Schulz's estate turned them down, believing A Charlie Brown Christmas to be perfectly fine without them. To be fair, they would undermine the anti-commercialism message of the story.
The 21st-century showings on ABC (which generally broadcasts both the cut version and the uncut version over a given holiday season) have taken further cuts, removing such iconic moments as Lucy pestering Schroeder over the proper playing of "Jingle Bells", to even cutting poor Shermy's only line.
The show is often paired with the much shorter Charlie Brown's Christmas Tales so that it can be shown intact and allowed to run over time.
Executive Meddling: Ultimately averted. CBS executives had issues with the special as it was originally put together. They objected to
The Scripture quotation that Linus recites to explain the meaning of Christmas.
Using actual children to voice the Peanuts characters.
In other words, nearly everything that makes this program a timeless classic. Fortunately, Charles Schulz, Lee Mendelson, and Bill Melendez stuck to their guns and were vindicated the moment it aired. note It also helped that they were smack up against a set-in-stone deadline, and Coca-Cola was satisfied with the special (it would have probably cost CBS more at that point to not air it, as that would jeopardize sponsorships).
Magnum Opus Dissonance: Nobody behind the project thought it was any good when they finished; Lee Mendelson and the rest of the team felt they "ruined Charlie Brown" when they looked at the final cut before it aired.
It apparently took a while for Charles Schulz himself to realize how iconic this special had become. In the late 1980's he fretted that he hadn't produced his "own Citizen Kane" and invested a good deal of time and money on his intended masterpiece, the flop live-action/animated combo It's the Girl in the Red Truck, Charlie Brown. Any fan could've told him he'd already made his Citizen Kane way back in 1965. (For added irony, Orson Welles didn't consider the actual Citizen Kane to be his masterpiece either)
Network to the Rescue: CBS made some cuts to the special in the '90s because shows made more room for commercials by then (see Edited for Syndication above). When ABC acquired the rights in 2001, they blocked out a full hour for the special so that it could run uncut* (aside from the aforementioned Coca-Cola references), commissioning Charlie Brown's Christmas Tales to fill the extra time. Note that ABC also airs a cut version during the season, but there will be at least one full-length airing.
Not Screened for Critics: CBS was so disappointed in A Charlie Brown Christmas before its first airing that they refused to let any TV critics see it beforehand, terrified that the inevitable avalanche of bad reviews would sink everyone's careers. They were eventually forced to relent in recognition of what happens when this trope is played, and let one writer from Time Magazine in he watched the special at CBS's office and then left without saying a word. Initially, this vindicated CBS's decision, but when the next issue of Time came out, the critic's review of the special was glowing.
Off-Model: Several animation errors exist, thanks in no small part to its cheap and rushed production.
One that Schulz himself often pointed out was that Charlie Brown's tree inexplicably grows a few branches between its introduction and the point where Charlie "kills" it.
The words on Lucy's "Psychiatric Help 5¢" stand change twice in less than a minute.
The Red Stapler: Inversion: The special almost single-handedly destroyed the market for Aluminum Christmas Trees so completely that later generations have to be reminded that such a thing actually existed.
What Could Have Been: When CBS ordered this special, they wanted a lighthearted goofy cartoon with the Peanuts characters that just happened to take place at Christmas time (evidently the suits hadn't read the comics very closely). They weren't expecting the potshots at commercialism or Linus's Bible recitation. It's a good thing the special didn't go this direction, because it wouldn't be nearly so memorable otherwise.
The slam against commercialism and the Bible recitation were only two of what the suits perceived as a laundry list of "issues" with the show. Others were the lack of a Laugh Track (standard equipment for comedy shows and cartoons at the time see Hanna-Barbera's output from this era), the use of actual children for the voice acting instead of adults, and the Vince Guaraldi jazz score. (The tune "Linus and Lucy" became an icon of the franchise, and Vince scored 17 otherPeanuts specials and the feature film A Boy Named Charlie Brown before his death). Thank goodness Schulz and Melendez stood their ground.
"Charlie Brown's Christmas Tales" seems to fit the idea of what CBS expected. So in a way, ABC got what CBS originally paid for.
Dave Brubeck and Cal Tjader both turned down offers to do the music, because they were too busy. Brubeck later did the score for This is America, Charlie Brown: The NASA Space Station.