MouseOver issue with Collider on prefab

I'm having the following problem. I have instantiated a number (~ 80) of sphere prefabs into a plane perpendicular to the camera (psuedo-2D). The prefab has an attached sphere collider and a script for handling the mouse over.

The mouse over script is working more or less flawlessly for 79 out of 80. However, one of these prefabs (distinguishable only by its distance from the others) is not working. This outlier registers a mouseover event any time the mouse is anywhere in the lower half of the screen.

The really strange thing is that the area effected by this "bug" changes with the scale of sphere along the y axis (direction towards the camera). Such that flattening the sphere decreases this effect. However, I can't just flatten the prefabs or they won't render right.

I can't see anything to differentiate this outlying prefab. I don't think it's my code, but rather that there is some crazy interaction between the collider and the mouseover function that I don't understand.

Thanks for your help.