RigidBody moving automatically to upper left in Space Shooter tutorial - unity3d

Question

I am following the space shooter tutorial on Unity website.
I have completed upto movement actions of the player object.
When I start my game, the spaceship automatomatically moves to the upper left corner even when no input is given.
I have followed the tutorial exactly as it is. Even the completed scene that is available in the Asset Store is having the same problem.
I am using Unity 5.3.
PlayerController.cs
using UnityEngine;
using System.Collections;
[System.Serializable]
public class Boundary
{
public float xMin, xMax, zMin, zMax;
}
public class PlayerController : MonoBehaviour {
public float speed;
public Boundary boundary;
public float tilt;
// Use this for initialization
void FixedUpdate()
{
float moveHorizontal = Input.GetAxis("Horizontal");
float moveVertical = Input.GetAxis("Vertical");
Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
GetComponent<Rigidbody>().velocity = movement * speed;
GetComponent<Rigidbody>().position = new Vector3(
Mathf.Clamp(GetComponent<Rigidbody>().position.x,boundary.xMin, boundary.xMax),
0.0f,
Mathf.Clamp(GetComponent<Rigidbody>().position.z, boundary.zMin, boundary.zMax));
GetComponent<Rigidbody>().rotation = Quaternion.Euler(0,0, GetComponent<Rigidbody>().velocity.x * -tilt);
}
}

Answer 1

Your code seems correct and since you say that the demo scene does the same, I suppose that the problem comes from your Axis input: The lines taht add movement
float moveHorizontal = Input.GetAxis("Horizontal");
float moveVertical = Input.GetAxis("Vertical");
Uses the Axes called Horizontal and Vertical. It is possible that on your instance of unity, these input are linked to a device that sends events (you maybe have a controller plugged...)
To test this, You can add the following line below reading the input:
float moveHorizontal = Input.GetAxis("Horizontal");
float moveVertical = Input.GetAxis("Vertical");
Debug.Log("Movement: " + moveHorizontal + ", " + moveVertical); // <-- add this
This will write the values you get as input. If you don't touch anything, they should be zero. If they are not zero, Go to Edit -> Project Settings -> Input, You will see how your keyboard, mouse and other controllers are linked to events in Unity such as Horizontal and Vertical
See http://docs.unity3d.com/Manual/class-InputManager.html for more info on Input Manager
Good luck!