Handles.Label makes the Camera draws the Handles by perspective mode, and the other Handles.* call
// correct, draw a line from (0,0,10) to (10, 0, 10)
Handles.DrawCamera(new Rect(0.0f, 0.0f, 960.0f, 640.0f), myCamera, DrawCameraMode.LightmapResolution);
Handles.color = new Color(0.0f, 1.0f, 1.0f, 0.6f);
Handles.DrawLine(new Vector3(0, 0, 10f), new Vector3(10f, 0, 10f));
// incorrect
Handles.DrawCamera(new Rect(0.0f, 0.0f, 960.0f, 640.0f), myCamera, DrawCameraMode.LightmapResolution);
Handles.color = new Color(0.0f, 1.0f, 1.0f, 0.6f);
Handles.Label(new Vector3(0, 0, 10f), "10"); // the only different
Handles.DrawLine(new Vector3(0, 0, 10f), new Vector3(10f, 0, 10f)); // even this line is broken