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.
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.
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).
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.
Anubis shifting hosts as they became too sick to use meant that four different actors (David Palffy, Gavin Hood, Dean Aylesworth, and Rik Kiviaho) played him on the lower planes. Yet a fifth actor, George Dzundza, played him as Jim in the Astral Diner in "Threads".
Three different girls played the growing Adria during "Flesh and Blood," then Morena Baccarin took over the role permanently in "Counterstrike".
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.
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.
What Could Have Been: 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.
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.