1. In mission editor, place the player as a unit with a bipod weapon (for example a BLUFOR NATO Autorifleman). To make bug most apparent, the player's unit should be located on flat ground, facing some terrain or objects that are above the plane of the local ground (for example, part way up a ridge, facing the hill on the opposite side of valley)
2. Place a subordinate soldier of any type near the player unit.
3. Preview mission. Turn on crosshair in game options.
4. Go prone.
5. Switch to optics.
6. Press F2 to select your subordinate unit. A cursor appears on screen. Move the mouse around, noticing the cursor follows the weapon aimpoint (centre of the screen).
7. Deploy weapon. Notice mouse cursor no longer follows the centre of the screen, and does not move off the plane in which the weapon is deployed (dictated by terrain slope).
8. Switch out of optics mode. Note mouse cursor again follows the centre of the screen as expected.