I personnally have spent a lot of wasted time on my Rubik's Cube. And through the many minutes/hours spent on it I had discovered that two adjacent sides control all the other ones. If two adjacent sides are correct, than the whole cube is correct.

Therefore, it is possible to get two opposite sides completed without completing the cube, but a third completed side would automatically be adjacent to both opposite sides, so that side could not be completed unless the cube is.

So,

on a side note, I think there are 1.69267*10^13 different combinations of a Rubik's Cube.