Focus-recompose with an aperture wider than f/2 and a close subject = back focus. That's simple geometry - the sensor is flat, and the lens field is flat (or it's supposed to be, at least), but when you focus-recompose, you move the camera in an arc, meaning you've decreased the distance to the subject, meaning the plane of critical focus ends up behind where you want it. With a deeper Dof, or if you don't print large (or look too closely), the effect is masked. But with a shallow DoF, it's an issue not having a reliable AF point at least close to where you actually need it.
The 5DII has the worst AF point spread of any current Canon body (relative to the frame) - even the entry-level Rebel/xxxD/xxxxD bodies have an AF point closer to the rule-of-thirds intersections. The center point of the 5DII is good, but it's usually not where you need it, and the outer points of the 5DII, besides having a rather pathetic distribution, aren't very reliable.
A 61-pt AF in the 5DIII, even if it's a simplified version of what's in the 1D X, will be most welcome!