Steps to reproduce

Start Assetforge

Add a primative cube to 0,0,0

Press the mirror button

Grab the top movement handle on the cube and try to move it upwards. 

I thought it might've been because the cube ends up below the grid when you do this, but the same problem occurs if you create the cube, move it up 2.0 units, mirror, then try to drag the up handle.

You can unlock it by clicking around (on and off the cube) a few times.