Colonel Samantha Carter speculated on the science of this ability. That yes, if a person could know the position and velocity of every particle in the universe at any given moment, they could accurately predict all of their interactions for the rest of time. But quantum mechanics blows that out of the water. According to the Heisenberg uncertainty principle, it is impossible to look at a sub-atomic particle and know both where it is and where it's going at the same time. The more accurately you fix its position, the more uncertain you make its velocity and vice versa. The best we can do is calculate probabilities. Thus, precognition likely only involves seeing probable futures. Therefore, some events seen may not actually come to fruition.
- Precognition on Wikipedia