AccurateShooter has an excellent post on the effects of canted scope reticles (when the cross-hairs are not aligned with the scope).

There was widespread agreement that too many scopes are leaving the factory with canted reticles, i.e. cross-hairs that are not plumb with the turrets. In fact, to my surprise, most of the top shooters I polled said, yes they have, at least once, purchased a $700+ scope from a major manufacturer that arrived with a canted reticle. The amount of cant ranged from an estimated one to three degrees.

More here.