Accidentally Accurate: In "Orpheus" Jack mans a sniper rifle during a rescue operation against a Goa'uld POW Camp. He aims for center of mass. Falls here because the showrunners' reasoning for doing that was because headshots are messy and they didn't want to have to argue with the network censors, but trained snipers in real life aim for center of mass because it's an easier target.
Amanda Tapping played multiple Carters in several different episodes. "Point of View" has Dr. Samantha Carter, "Gemini" features extended interactions between Carter and RepliCarter, and "Ripple Effect" features over a dozen Carters in a single scene.
"Tin Man", where the alien Harlan creates robotic duplicates of SG-1. They reappear in "Double Jeopardy", where the two O'Neills get into a fistfight.
Guest star Dom De Luise (briefly) as the title character and his creator in "Urgo".
Starting in season nine, Ba'al began cloning himself, with Cliff Simon playing multiple Ba'als in several episodes.
The show was fond of making MacGyver references — which of course, leads to Celebrity Paradox. Lampshaded by Amanda Tapping in a blooper shown in the 200th episode special "Inside the 200th Episode". O'Neill is from Minnesota, the same state MacGyver is from (also Richard Dean Anderson's home state). Like MacGyver, O'Neill is an astronomy enthusiast (MacGyver was shown living in an observatory in the pilot of his TV show). An inversion: Jack likes having a gun and will protest whenever he's told to not carry one, while MacGyver did not like guns and was known for avoiding their use. Like MacGyver, O'Neill is an avid fisherman. Both MacGyver and O'Neill are shown to be hockey fans.
John Billinsley guest-starred in the season six episode "The Other Guys" as Dr. Coombs, who was a profound Trekkie. John Billinsley was at the same time starring as Dr. Phlox on Star Trek: Enterprise.
Patrick McKenna guest-starred in "The Other Guys" and "Avenger 2.0" as Dr. Jay Felger. In the latter episode he was seen putting a roll of duct tape into his backpack. DVD commentary confirmed this was a Shout-Out to The Red Green Show, where McKenna played Harold.
When Vala is pitching story ideas to Martin Lloyd in "200", he immediately recognizes her pitches as retreads of classic stories, including The Wizard of Oz and Gilligan's Island. He recommends that, if she is going to rip off something, make sure it is something nobody has ever heard of. Her next suggestion is Farscape, the series she (Claudia Black) and Mitchell (Ben Browder) had previously starred in before joining SG-1, with Claudia reprising her role as Aeryn Sun (Browder did not appear as John Crichton, but instead as Stark, while Michael Shanks was Crichton, in a meta-reference to the resemblance between the two actors). Martin admits that he had never heard of that one.
When the one-off character Alec Colson, played by Charles Shaughnessy, enters the Alpha Site the first person he's introduced to is one Captain Sheffield.
Dwight Schultz plays The Gamekeeper who runs a virtual reality simulation and insists that the inhabitants never leave it. On Star Trek: The Next Generation, Schultz played Lt. Reginald Barclay who spent all his free time on the holodeck and was afraid to leave it.
Ascended Fanon: Originally Jack and Sam were never supposed to develop romantic feelings for each other. When the Jack×Sam shipping started in the fandom, the writers noticed that they had the chemistry for it and threw it in.
Backed by the Pentagon: Particularly the US Air Force. Multiple real-life Air Force Chiefs of Staff have appeared on the show, playing themselves. Other branches became involved on occasion, and the US Navy loaned them a submarine for "Small Victories." And again for the movie "Continuum".
The text on the back of the DVD cover of the Swedish release of "Children of the Gods" (DVD release of the pilot episode) claims that the villains of the movie are Ra, "the brutal Goa'uld", and General Hammond. This is very not correct.
Defictionalization: There is a door in the real-life Cheyenne Mountain Complex labelled "Stargate Command", which some sources say has six locks and a guard placed next to it. Whether or not the locks and guard are really there, the door itself leads to a broom closet.
Directed by Cast Member: Amanda Tapping, Michael Shanks, and Christopher Judge all wrote or directed episodes, and Ben Browder shared a story credit for one episode.
Doing It for the Art: One of the "flashbacks" from "200" features an incident where O'Neill was rendered invisible; one of the scenes features him walking down a hallway and talking with Teal'c, but since he is invisible he can only be located by the coffeee cup that he is carrying at the time. The plan was to have one of Richard Dean Anderson's stand-ins or stunt-doubles actually perform the scene, since it would require wearing a full-body green suit in order to have the body edited out in post production, but RDA insisted on actually performing the scene himself. You still cannot see him, since he is invisible, but hopefully you can just tell that it is really O'Neill walking with Teal'c.
In "Affinity", Lois Lane from Smallville is Teal'c's neighbor.
Wayne Brady plays a Jaffa in the season 8 episode "It's Good to Be King". Guess Dave was right about Wayne Brady's dark side.
Bob is the diner owner in season 10's "Memento Mori" when Vala loses her memory.
Don S. Davis appeared in at least two episodes of MacGyver. The first appearance was as a cement truck driver who lent his truck to Mac in his latest disaster relief, and the second as a hunter trying to track down Mac in order to silence him about illegal hunting.
Old Shame: Not the series itself, but the disintegration feature of Zats. The writers have frequently mentioned in episode commentaries what a bad idea that was, to the point where they mock the it in-universe in "Wormhole X-treme!"
SG-1 recast every character from the original Stargate with the exception of Daniel's brother-in-law Skaara (Alexis Cruz) and father-in-law Kasuf (Erick Avari). Lampshaded when O'Neill is mistaken for another soldier with the same name (except that it's spelled O'Neil) and he remarks that the other one had no sense of humor.
Elizabeth Weir was played by Jessica Steen in "Lost City", then was replaced by Torri Higginson for "The New Order", Stargate Atlantis and Season 10's crossover "The Pegasus Project". Michelle Morgan played her during later events in Atlantis, though this instance was given a Hand Wave.
Drey'auc was played by Salli Richardson in "Bloodlines" before being replaced by Brook Parker in "Family" and "Threshold".
An odd semi-example: When Apophis is masked while under the alias Na'onak, he is played by Dion Johnstone instead of Peter Williams.
Vaitiare Bandera (Sha're) was married to Michael Shanks (Daniel Jackson) in real life, and was pregnant with his child when her character was written as carrying Apophis' son.
Michael Shanks got appendicitis during the filming of "Nemesis", so Daniel Jackson was written out of most of the action in that episode and "Small Victories" by also giving him appendicitis.
Carter temporarily changed her primary weapon from the P90 to the custom-built "Carter Special" starting in the middle of season seven because the show's suppliers ran short of P90 blanks during the invasion of Iraq.
At a convention panel, actor Don S. Davis (General Hammond) was asked what role he would play if given a choice of switching characters, and he responded that he would play his own role since he had the best chair on the set. When Hammond returned in "Prometheus Unbound" after he was promoted away from the SGC, he made a point of taking the chair with him when boarded the Prometheus.
Vala's pregnancy in season nine stemmed from Claudia Black's real-life pregnancy.
Sergeant Siler is routinely battered throughout the series, from being a test target for new staff-weapon armor, to having a DHD blow up in his face. At first glance it's just a running gag, like Daniel dying and coming back to life all the time, but then you see the credits and find out that Dan Shea, the actor that portrays the good Sergeant, is also the stunt coordinator. Also, Shea was Richard Dean Anderson's stunt double in both MacGyver and SG-1.
Real-Life Relative: Lexa Doig, who played Dr. Carolyn Lam in season 9 and 10 is the wife of Michael Shanks, who plays Daniel.
Talking to Himself: Michael Shanks voices several Asgard, including Thor, so whenever one of them and Daniel Jackson are talking to one another, it's an example of this. Lampshaded when speaking to Kvasir, Daniel dryly comments he misses Thor.
Most of Jack O'Neill's snarking for the first couple of seasons was improvised by Richard Dean Anderson, after which the writers gave up and started writing it into the scripts.
Samantha Carter's line in "Children of the Gods" about how the SGC's gate control computer had been MacGyvered from three supercomputers was an ad-lib Amanda Tapping came up with during her audition.
The concept of Jaffa performing a meditation called "kel'no'reem" was apparently inspired by Christopher Judge falling asleep on set during a take, and Michael Shanks quipping "Oh, he's not sleeping, he's meditating."
The Romance Arc between Jack and Sam started with an exchange in "Solitudes" that was ad-libbed by RDA and Tapping. Trapped in an Antarctic ice cave, they snuggle together for warmth.
From season six through season eight, the showrunners were in constant fear of cancellation and, unlike previously where the season finale ended on a cliffhanger, closed out the season's arc solidly. And yet Sci Fi Channel kept renewing it, continually derailing plans for a Grand Finale film (a fact referenced by "200"). And then just as the showrunners decided they could keep going, the network ended the series.
In-Universe, Wormhole X-Treme! was picked up for a movie in "200", five years after being cancelled after the third episode. Then the network dropped the movie and started the series back up again, and it ran for ten seasons before getting a green light on the movie again.
Apparently, there were plans to make Adria more long-lasting, including having her fuse with a Goa'uld symbiote. However, plans fell through after Sci-Fi ended the series, and instead had her do a re-hash of Anubis's final plot. The Goa'uld symbiote part actually happened in abbreviated form, resulting in Adria having to Ascend to a Higher Plane of Existence after a failed extraction attempt killed the symbiote but left her dying.
After the series ended the plan was to do one or two movies a year for both SG-1 and Stargate Atlantis, but according to the producers the bottom fell out of the DVD market and Sci-Fi balked at continuing after the first two. Apparently the third movie, entitled Stargate: Revolution, would have featured Jack's return as the main character and it would have finally had the Stargate program going public. However it can be considered shelved for good at this point.
With Lyrics: The theme song was given lyrics in one DVD commentary, sung by the producers.
Word of God: The cast has at times spoken about what they think was going on between the scenes and time-skips in the series finale, "Unending". In particular, Amanda Tapping and Christopher Judge said they played their interactions to imply Carter and Teal'c had developed a relationship, and either Claudia Black or Michael Shanks said the reason Vala's crying in one scene is because she miscarried.
Word of Saint Paul: The finale of the series contains a long Time Passes Montage with little to no dialogue throughout. Several of the actors involved have given their take on what happened — such as Amanda Tapping and Chris Judge saying they played their parts as if Sam and Teal'c had developed a relationship, and Michael Shanks saying the reason that Vala is crying and being consoled by Daniel in one scene is because she miscarried.
In "Window of Opportunity" a minimum number of the Groundhog Day Loops experienced can be calculated. At the end it is said the Tok'ra have been trying to reach them for over three months. No set values are given, and we don't know when the Tok'ra began to try to contact Earth. With this, considering an average of 30 days to a month: 30(3 months)= 90 days. 90(24 hours)=2,160 hours. About ten or so hours per loop, so 2,160/10=216. Teal'c and O'Neill experienced a minimum of 216 loops, probably many more.
The Goa'uld bear many similarities to a creature encountered by the Enterprise in Star Trek: The Next Generation in season one: a worm-like creature that attaches to the spinal cord, cannot be removed, grants superhuman strength and assumes control of the host body.