How to reproduce:

1. Open a new project
2. Create a script named GetMaskBug.cs with the following contents:

using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
public class GetMaskBug : MonoBehaviour {

public LayerMask defaultMask;
public LayerMask ignoreRaycastMask;
public LayerMask waterMask;

void OnValidate () {
defaultMask = LayerMask.GetMask("Default");
ignoreRaycastMask = LayerMask.GetMask("Ignore Raycast");
waterMask = LayerMask.GetMask ("Water");
}

}

3. Save the script and return to the editor
4. Attach the script to any object in the scene
- Note how the Default Mask property says "Nothing" while the rest appear to work fine
- This also happens when using LayerMask.GetMask(LayerMask.LayerToName(0))