物体有时穿过平面

我正在学习使用Unity，我在课程中看到的一个小游戏遇到了问题。是迷宫游戏和球的地方，你必须移动迷宫才能把球带到球门。问题是，球有时会穿过迷宫的平面，落入虚空。

using UnityEngine;
using System.Collections;
public class controllaberinto : MonoBehaviour 
{
    public enum TGameState
    {
        PLAYING=0,
        END_GAME
    }
    public float RotationalSpeed;
    public GameObject Ball;
    Vector3 StartPosition;
    public TGameState GameState;
    void Start () 
    {
        StartPosition = Ball.transform.position;    
    }   
    void Update () 
    {
        switch (GameState) 
        {
        case TGameState.PLAYING:
            UpdatePlayingGameState();
            break;
        }   
    }
    void UpdatePlayingGameState()
    {
        if(Input.GetKey(KeyCode.A))
           transform.Rotate(new Vector3(0.0f, 0.0f, RotationalSpeed*Time.deltaTime));
        if(Input.GetKey(KeyCode.D))
           transform.Rotate(new Vector3(0.0f, 0.0f, -RotationalSpeed*Time.deltaTime));
        if(Input.GetKey(KeyCode.W))
           transform.Rotate(new Vector3(RotationalSpeed*Time.deltaTime, 0.0f, 0.0f));
        if(Input.GetKey(KeyCode.S))
           transform.Rotate(new Vector3(-RotationalSpeed*Time.deltaTime, 0.0f, 0.0f));
    }
}