I changed your little scene a bit, just to make it more intuitive for me.
I don't know what happens, but it has to do imo with the relative, combined 
positions of camera, media sphere, and spotlight. I don't think that the 
spotlight code itself is the culprit. Try this with the scene below, only 
switching the camera z-position between -70 (no aberration) and -80 
(aberration in one of the spheres).

Thomas

camera {
  perspective
  location  <20,25,-70>
  look_at   <0,0,0>
  right     x*image_width/image_height
  angle 90
}

light_source {
  0*x
  color <1,1,0>
  spotlight
  translate <0, 36, -36>
  point_at <0,0,0>
  radius 7
  tightness 10
  falloff 9
  media_interaction on
}

// floor
object{
        box { <-65,-1, -40>, <65,0,40>
        texture{
                pigment{agate scale 15}
                finish {
                        ambient .2
                        diffuse .8
                       }
                }
            }
       }


// oblong box
box{<-4,0,-4>, <4,30,4>
        texture{
                pigment {color rgb 1}
                finish{
                        ambient .1
                        diffuse .9
                      }

               }
     translate <0,0,0>
    }

#declare media_sphere =
sphere{0,1 hollow no_shadow
    texture{
        pigment{color rgbt 1}
        finish{ // not needed, but does not alter problem
                ambient 0
                diffuse 0
              }
           }

    interior{
         media{
                emission .4
                density{bozo}
              }
            }
      }

object{media_sphere translate <-10,20,-50>}

object{media_sphere translate <10,20,-50>}

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