Lifting Mice From Desk Doesn't Cause Pointer To Move

A user called in to say her mouse was only moving left and right and not up and down. This was a common fault with that particular model of mouse, so I brought a replacement over, swapped it out, and she tried it out.

"Nope, same problem," she said, and showed me. For left and right movements, it worked fine, but when she moved the mouse up and down that is, physically off the desk and into the air it just didn't work at all.