Scripting Reference
Debug.LogWarning

static function LogWarning (message : object) : void

Description

A variant of Debug.Log that logs a warning message to the console.

JavaScript 
Debug.LogWarning("Watch out!, this is a warning!");
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        Debug.LogWarning("Watch out!, this is a warning!");
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        Debug.LogWarning('Watch out!, this is a warning!')

static function LogWarning (message : object, context : Object) : void

Description

A variant of Debug.Log that logs a warning message to the console.

When you select the message in the console a connection to the context object will be drawn. This is very useful if you want know on which object a warning occurs.

JavaScript 
Debug.LogWarning("A warning assigned to this transform!", transform);
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        Debug.LogWarning("A warning assigned to this transform!", transform);
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        Debug.LogWarning('A warning assigned to this transform!', transform)