1) Dark shadows
2) But mainly burnt out highlights, even very little zones
(cmyk 0,0,0,0 - RGB 255,255,255 - % 100,100,100).
These will become "paper color" if you print it and it is not good.
For offset printing (magazines, catalogs, etc) the white should not exceed 95-97% (3-5% if you measure in cmyk)
(I am not speaking of an "isolation white", this one must be 100% white)