I dont really know how or when it happens, but sometimes when I aim with my weapon but not down the sights my character aims downward but the crosshair is still centered.
Now when I aim down the sight he is aiming very low like knee height but without moving the mouse, so the crosshair is centered but the gun is not.
What fixes this issue is usually a relog or I got lucky when switching weapons.
If you don't notice it early enough you could be aiming at an enemy with the crosshair on him but when you aim downsight the gun is in a different state (aiming at the ground).