The quantum mirror that had been discovered on P3R-233 and had drawn Daniel into a parallel universe was brought back to Earth and stored at Area 51 until it again became a doorway between worlds.
Carter and Kawalsky arrived through the mirror from yet another alternate reality. In their version of Earth, the SGC was the SGA, Daniel was not with the program, Kawalsky was alive, and Dr. Carter was a civilian who had married O'Neill a year earlier. The Goa'uld had taken over every major city on Earth and had made slaves of the population, Teal'c was the First Prime of Apophis, leading the attack, and O'Neill had been killed in the battle.
Due to temporal entropic cascade failure, in which duplicate people existing in the same reality cause a temporal distortion, Carter was not able to remain in this reality with her own counterpart. Instead, SG-1 returned with her through the mirror to seek the aid of the Asgard in the alternate universe, in order to defeat Apophis and save Earth from the Goa'uld attack. Carter and Kawalsky remained behind, as did the remote control for the mirror. Concerned about future threats from the quantum mirror, Hammond ordered that the mirror be destroyed following the mission.
Cross Reference: Alternate Reality, Alternate Reality - P3R-233, Alternate Reality - Ripple Effect, Alternate Reality - Universal Bridge, Energy Generator, Entropic Cascade Failure, Charles Kawalsky, Quantum Mirror
Episode Reference: Point of View