Hi,a Problem When I Save a Prefab

Hi,When I save a prefab(Drag the GameObject to Project view),The editor show many of this exception.

SecurityException: Reading from the screen is not allowed when you have used a downloaded texture without proper crossdomain.xml authorization UnityEngine.Texture2D.ReadPixels (Rect source, Int32 destX, Int32 destY) UnityEditor.PreviewRenderUtility.EndStaticPreview () UnityEditor.GameObjectInspector.RenderStaticPreview (System.String assetPath, UnityEngine.Object[] subAssets, Int32 width, Int32 height) UnityEditor.AssetPreviewUpdater.CreatePreviewForAsset (UnityEngine.Object obj, UnityEngine.Object[] subAssets, System.String assetPath) UnityEditor.DockArea:OnGUI()

I don't know what wrong I did,But I'm sure I it not anything download in my game object. Is it a but of Unity3d?