Beam Me Up, Scotty!: Cookie Monster has never used the catchphrase "Cookies are a sometimes food!" It was Hoots the Owl who sang "A Cookie Is a Sometimes Food" to Cookie Monster. After the song, Cookie Monster replied "Me get it, cookie is sometimes food. You know what? Right now is sometime!" and devoured the cookie.
The Danza: Bob McGrath, who plays Bob Johnson on the show.
Linda Bove As Herself, and Miles Robinson was originally played by Miles Orman.
Sesame Street has had a lot of Danzas. Even Tony himself has appeared as a guest star.
Defictionalization: Outside Philadelphia, there's a theme park in Sesame Place. It includes a perfect life-size replica of the set of the show, and the characters come out to greet guests constantly. Yes, you can take photos.
For the show's 40th anniversary, a corner of Manhattan was temporarily renamed 123 Sesame Street.
Enforced Method Acting: Subverted in "Goodbye Mister Hooper". The cast did fine in rehearsal, but when the cameras rolled, they barely hung on, and Loretta Long (who played Susan) flubbed a line. Director Jon Stone wanted to try again, but it was too much for the cast, and so executive producer Dulcy Singer decided to keep the scene as is. The footage that the crew kept was the scene that we saw ... and it showed, quite candidly, that even adults cry and genuinely feel deeply saddened when a person they were close to has died, and that children aren't the only ones who become emotional.
Missing Episode: The late 1970s episode where Margaret Hamilton reprised◊ her role as The Wicked Witch of the West is currently lost. The one time it aired, numerous parents sent hate mail saying it was too frightening for their children, and at least one Wiccan mother complained that the episode represented negative stereotypes of witches.
The episode "Snuffy's Parents Get a Divorce" never aired as the test audiences didn't seem to get the concept well. Only one known picture◊ exists of it.
Missing Sketch: Regarding massive criticism, Katy Perry's "Hot and Cold" segment with Elmo has never been aired on television, due to claims that Perry's dress was too risque for a preschool to kindergartner-aged educational TV show. While the dress did show some cleavage (behind a mesh panel) and seemed to accentuate her chest in a way that makes it look as if the dress doesn't fit her, most viewers who saw the original sketch on YouTube declared that the ban is yet another sign of parents overreacting to sexuality and near-nudity on TV, yet turning a blind eye to violence, gore, and Nightmare Fuel.
Long-Runner Cast Turnover: During the nineties, a number of magnates passed away or began to appear less. Season 37 saw the start of another ongoing turnover.
Kevin Clash, Steve Whitmire, David Rudman, Stephanie D'Abruzzo, and a vast number of the Muppet performers prior to the first few seasons also fall into the same category as Spinney.
Same can be said for a number of the live actors who joined the cast within the last 20 years or so, including Alan Muraoka and Chris Knowings.
The Other Darrin: Gordon. And Miles. Some of the Muppets' performers were also replaced.
Throw It In: A small-scale version of Zoe was originally built for her role as "Mousey the Hatter Helper" in the direct-to-video Abby in Wonderland movie, but the puppeteers liked it so much that, starting in Season 40, they made this Zoe the de facto Zoe. Sesame Workshop, of course, tested this smaller Zoe by having kids visit the set, and they didn't seem to notice.
A good chunk of Muppet dialogue is ad-libbed, or at least used to be. Watch an old "People in Your Neighborhood" sketch to see Jim Henson try to make Bob crack up.
One recurring feature was having the Muppets interact with children in unscripted segments, resulting in such classic (and adorable) bits as this one.
What Could Have Been: Caroll Spinney was very close to being aboard the Challenger as Big Bird. But the expenses of sending him and the puppet suit into space were enough to keep him off the shuttle and its explosive fate.
The pilot episodes had the Muppets kept separate from the humans and the street, but since the kids paid more attention to the Muppets in test screenings, they were wisely integrated.
At one time there was an episode that was supposed to air on September 27, 2010, in which Katy Perry performs a version of "Hot n Cold" with Elmo. However, because the song wasn't kid-friendly enough, and because of the outcry of Media Watchdog groups over her Impossibly-Low Neckline and Absolute Cleavage, Sesame Workshop had to pull the episode a few days before it was to air. You can still watch the video here.
Snuffy's parents were meant to get divorced but test audiences had a negative reaction to the episode. It's never been aired.