Mirror Modifier seemingly not exact I am currently modeling a character. The face has a mirror modifier on it (not applied yet), and the eyes have been mirrored (applied) and separated, with a lattice acting on each. The lattices have basically been mirrored, (I used Scale X -1), so I know they are the same. However, I noticed the eye sockets don't appear to match up horizontally. If you look at the picture, I have created a small green box that is mirrored, and you can see the difference in the eye socket. I also added the red lines for more reference points. If you follow the red lines and the green box from left to right, you will notice that the right eye socket appears to be shifted down.
I realized that somewhere in the modeling process, the skin and eye meshes had become very, very slightly rotated. This meant that my mirroring was also rotated.