Parallax problems result from the target image projected from the objective not being coplanar with the reticle. If the target and the reticle are not coplanar (i.e. focal plane of the target is either in front of or behind the reticle), when the shooter's pupil position changes (often due to small alterations in head alignment) behind the eyepiece, the target will produce a different parallax to the reticle image. This parallax difference will produce an apparent movement of the reticle "floating" over the target, known as the "parallax shift". This optical effect causes aiming errors that can make a shooter miss a small target at a distance, because he/she is actually aiming at a different spot to the assumed point of aim. It can also lead to unreliabilities when zeroing the gun.