It happens when I move the camera. Sorry. I can't attach the .blend file.
I put 2 objects on another layer (Cube and this gun), then I made those objects to attach to the plain axis when I press a button. The axis is attached to single bone. My guess is that it 'calls' the object from the other layer in frame 1 but attaches to the axis in frame 2 when I already moved the camera which is a parent for the bone. But how do I fix that?
Bone's animation is 2 simple keyframes.