Load two 3D grayscale images so that you have in data manager whereby the images should be on an position so that they overlap. Then you have in data manager:
-> Img1 (Opacity = 1)
-> Img2 (Opacity = 1)
In 2D as well as in 3D multi widget planes the Img1 is painted first so that you cannot see Img2 at the overlap. Now change the opacity of Img2 from 1 to e.g. 0.8. Normally, on the overlay, Img1 should remain (works in 2D), but suddenly Img2 appears in 3D!!!