ENTROPIC CASCADE FAILURE
Temporal entropic cascade failure on the cellular level is a side effect of travel to an alternate reality. The increased entropy generated by duplicate people existing in the same reality causes temporal distortion, an effect with profound physical symptoms that are fatal if unchecked. When Samantha Carter from an alternate reality entered through the quantum mirror, the temporal distortion made it impossible for her to remain in a reality in which Carter already existed.
Six years later, an alternate SG-1 team used a black hole to create a rift in the space-time continuum which allowed multiple SG teams from different universes to cross into our reality. In this case, entropic cascade failure was not encountered, and the proximity of the various alternate realities in relation to each other may have accounted for the absence of the phenomenon.
Cross Reference: Alternate Reality, Alternate Reality - Quantum Mirror, Alternate Reality - Ripple Effect, Quantum Mirror
Episode Reference: Point of View, Ripple Effect