In "The Kindred Part 2", Beckett mentions feeling like "A Dog's Breakfast", a reference to the independent film Paul McGillion starred in with several other Stargate alumni, written by David Hewlett.
In "Brain Storm" Jennifer Keller, played by Jewel Staite, references her previous role (Kaylee of Firefly) and her fascination with strawberries.
Dr. Jennifer Keller: Seriously, have you tried these strawberries?
The Danza: Chuck the Technician, played by Chuck Campbell. He got his name when Torri Higgenson accidentally called him by his real name in a scene instead of his character's rank (Sergeant) and it was left in as Ascended Fanon.
Fake American: Most of the main cast are Canadians playing Americans. In the original cast, only Joe Flanigan (American playing an American) and David Hewlett (Canadian playing a Canadian) were actually playing their correct nationalities. They are, by coincidence, also the only characters to appear in every episode of the series.
Fake Brit: Fake Scotsman, specifically. While Paul McGillion himself is Canadian, he was born in Scotland to Scottish parents and lived there at various times during his childhood. His accent is affected however.
Fake Nationality: Sort of. Many of the actors were born in the countries that their characters are from, but they emigrated to Canada at young ages.
It's rare for fans to refer to the Genii leader by his real name rather than "O'Brien".
Michael is often conflated with Trip Tucker, and so on.
Mylène Dinh-Robic, who played "Dr. Olivia Fawcett" in "The Listener" also played as one of the Athosian prisoners in episode "The Kindred".
Nicole de Boer played Dr. Alison Porter in the episode "Whispers". She's the one who played Ezri Dax on Star Trek: Deep Space Nine. A young Nicole de Boer had also previously appeared alongside David Hewlett in Cube
Mr. Woolsey will be familiar to Trekkies as the Doctor (no, not that one), and Dr. Keller was played by Jewel Staite, who was also Kaylee on Firefly.
Incidentally, Jewel Staite also played the Wraith child Ellia in the episode "Instinct" in heavy makeup for the very reason of allowing her to possibly come back later as another character.
The Other Darrin: Elizabeth Weir was played by Jessica Steen during her first appearance on Stargate SG-1. Torri Higginson took over the role in the SG-1 Season 8 two-part premiere. Higginson was then replaced by Michelle Morgan as a Replicator version after the Asurans destroyed her body.
Real-Life Relative: David Hewlett's sister Kate Hewlett plays Rodney McKay's sister Jeannie Miller. In "Hot Zone", McKay was orginally supposed to mention he had a brother, but David Hewlett asked if he could change the line to sister, in the hopes that he could get his real-life sister to star alongside him.
When Col. Samantha Carter leaves SG-1 to become leader of the Atlantis Expedition, she says, "I know I am going to miss this place, and all of you," to old friend Teal'c. The tears in Amanda Tapping's eyes were likely not faked; she herself was saying goodbye to SG-1 after ten years.
Jason Momoa got a large tattoo on his left forearm between the filming of seasons two and three, forcing Ronon to likewise get a tattoo in the series. This was applied in "Reunion", where the application apparently hurt and forced Ronon to lightly slap the person doing the inking; Jason had not told the producers about the tattoo, and they were unhappy to learn about it after the fact.
Teyla became pregnant in the fourth season when Rachel Luttrell became pregnant in real life.
Amelia Banks, one of the recurring control room technicians, is a proficient student of kickboxing. Her actress, Sharon Taylor, is a kickboxer in real life, and when the producers learned this about her they wrote it into the character.
Throw It In: After Torri Higginson accidentally called the Gate Technician by his real name, "Chuck", the producers decided to keep it in and make it has official name, since he had not had one until that point.