This script is parented to the arm gameobject to make so that when you move your mouse the arms move a little ahead of the camera and then settle back to be aligned with the camera. The problem is that when I move my mouse the arms flip randomly between what I want and being randomly flipped 180 degrees around the Y-axis. This is a modified MouseLook script from standard assets.

using UnityEngine;
using System.Collections;

[AddComponentMenu("Camera-Control/Mouse Sway")]
public class MouseSway : MonoBehaviour {

    public float sensitivityX = 1F;
    public float sensitivityY = 1F;
    public float offsetY = 0;

    public float minimumX = -30F;
    public float maximumX = 30F;

    public float minimumY = -30F;
    public float maximumY = 30F;

    float rotationX = 0F;
    float rotationY = 0F;

    float tSinceLastMoveX = 0F;
    float tSinceLastMoveY = 0F;

    Quaternion originalRotation;

    void FixedUpdate ()
    {

         // Read the mouse input axis
         rotationX += Input.GetAxis("Mouse X") * sensitivityX;
         rotationY += Input.GetAxis("Mouse Y") * sensitivityY;

         rotationX = ClampAngle (rotationX, minimumX, maximumX);
         rotationY = ClampAngle (rotationY, minimumY, maximumY);

         Quaternion xQuaternion = Quaternion.AngleAxis (rotationX, Vector3.up);
         Quaternion yQuaternion = Quaternion.AngleAxis (rotationY, Vector3.left);

         transform.localRotation = originalRotation * xQuaternion * yQuaternion;
       offsetY = 0;
    }

    void Start ()
    {
       // Make the rigid body not change rotation
       if (rigidbody)
         rigidbody.freezeRotation = true;
       originalRotation = transform.localRotation;
    }

    public static float ClampAngle (float angle, float min, float max)
    {
       if (angle < -360F)
         angle += 360F;
       if (angle > 360F)
         angle -= 360F;
       return Mathf.Clamp (angle, min, max);
    }
}