Thank you, Lane.
No, I don't use any other input device: just the mouse and the keyboard. I usually do more things in each state, but when things produce unexpected results (as in this case) I tend to oversimplify in order to discard unnecessary clutter.
The original problem is to be able to pick up a World point using the mouse position, but I can't use Mouse Pick because there is not always some geometry to catch the raycast. As such, I use a fixed distance from the main camera to complete the Vector3 (which, by the way changes less than 1% between loops). So, since there isn't any easy way I am aware of to point-and-shoot, I am assuming that positionX and positionY in the screen, plus a fixed-distance point can help me to trace a targeting line, which can be extended to infinity if need be, will intersect my enemies regardless of their distance. When I use the ScreenToWorldPoint function to achieve this end, the input parameters are the correct X and Y coordinates of the screen, then I move a sphere to that precise World Point located 8 meters away from the main camera, but the Y component starts to change randomly and keeps going lower and lower, even if the Y screen coordinate remains at the top of the screen and the Y World component runs towards the negative realm.