At the very beginning of the episode you can clearly hear what Sam and Kawalsky are saying while on the other side of the mirror. However, later in the episode when Daniel connects to the universe with "Captain" Carter, no sound is transmitted through the mirror.
In this episode, Hammond and Dr. Carter said they found the quantum mirror on P3X-233, but it really is P3R-233.
At the end when Thor was coming to save Earth, his ship was detectable on radar but in earlier episodes they were undetectable to radar unless in the alternate reality the Asgard don't have that in their technology.
Jack: (about Asgard helping to defeat the Goa'ulds) Come on. Ya gotta love those guys!
(Jack bumps into Carter1 after talking to Carter2)
Jack: How are you doing with this... twin thing?
Carter1: Do you have a couple of hours?
Carter1: That was the answer.
Jack: Oh, oh.
Carter2: We made a good team, Major.
Carter1: Yeah, we did, Doctor.
Carter2: Plus your hair's kinda grown on me. Pardon the pun.
Sam: This is incredible. If the Asgard could design this to give the Gate extra juice, then they're just the little, green men we're looking for.
Jack: They're grey, actually. Roswell Grey to be exact.
Sam: This is hard, goodbye for a second time.
Jack: This is the first time.
Sam: It doesn't feel that way to me; you have to understand, my Jack had the same face, same voice... same hands.
Jack: Which brings to mind an obvious question: how could you marry such a loser?
Daniel: Well this can't be the Dr. Carter I met in the alternate reality. She's... dead?
Jack: All right, just to clarify. This Carter is from an alternate, alternate reality?
Jack: Does anybody else have a problem with that?
Teal'c: Which reality is actually real?
Jack: Thank you, Teal'c!
Apophis: My First Prime killed you before my very eyes!
Jack: Thanks, I'm feeling much better now.
Jack: Kawalsky, in your special ops training did they cover silence?
Jack: What the hell does 'Kree' mean?
Daniel: It means a lot of things. Loosely translated, 'attention', 'listen up', 'concentrate'.
Jack: 'Yoo hoo'?
Jack: Well, the combined IQ of Earth may go up a few points having two Carters around.
Teal'c: (to Teal'c 2) Renounce Apophis and I will let you live.
Teal'c 2: Shol'vah! (Teal'c shoots him)
Jack: The idea was to Zat him!
Teal'c: As I said before, ours is the only reality that matters.
Jack: Is he talking about that alternate reality thing that Daniel futzed with?
Jack: (to Carter2) For all we know, you could be her evil twin. But, then we'd be dealing with cliches, and you know how I feel about those. (points at Carter1) No, actually, you know how I feel about those.
"Our" Sam: Oh my God, this is too weird!
Jack: Yeah... how about that hair?
Jack: Ladies...Sams. We're all in this reality together.
Jack: Alright, so it's possible there's an alternate version of myself out there that actually understands what the hell you're talking about?
Peter also plays a Jaffa. When they're in the other reality and two Jaffa inform Apophis of the Asgard ship, he's the one in the background.
Director's Cameo: Peter DeLuise is the voice of the man interviewing Dr. Carter on the recording.
Dr. Carter and Maj. Kawalsky come from a different reality than the one Daniel went to; one thing that's mentioned is how there are only "a few" realities where Earth hasn't been overrun. We see at least one other where the Quantum Mirror is still in storage in Area 51... Also, Gen. Hammond says that he's going to "have the Quantum Mirror destroyed," but we all know how eager the crew at Area 51 are to get rid of useful technology. Perhaps it could have an iris attached to it? After all, visible light can travel through it when its activated, so its possible that radio waves can too, and we could have a "Quantum Mirror: QM-1" spin off show.
Syndication airdate: November 6, 2000.
In the alternate universe (where he did not turn), Teal'c wears a goatee. This is very likely a reference to the episode "Mirror, Mirror" of the original Star Trek series, where several members of the Enterprise crew are transported to an alternate reality where "evil" Spock wears a goatee.