To reproduce:
- Open attached project
- Open the TestScene Scene.
- Have the Game and Scene tabs open next to each other. Put the Scene view in 2D mode so that the entire Canvas is visible.
- In Edit > Project Settings > Player ensure that "Use Direct3D 11" is NOT checked.
- Select the child Image GameObject under the Canvas GameObject.
- Press "Play" and left-click and hold anywhere in the Game view and drag in any direction to create a marquee selection box.
- Notice in the Scene view how the position of the stretched image is off the RectTransform. This is done using localScale in the GUISelectionBox script.
- Stop the Player.
- ENABLE "Use Direct3D 11" in the Player settings.
- Make sure the child Image GameObject is selected.
- Press "Play" and create a selection box again. Notice how the image's positioning is now correct.

The scene works by the 3 scripts on the "Managers" GameObject. It fires EventManager events from the InputManager. The GUISelectionBox listens for those events, and creates the selection box accordingly.

Link to forum thread: http://forum.unity3d.com/threads/rts-style-drag-selection-box.265739/#post-1792706