A slanted picture of a cube does not look like a cube despite the presence of depth cues (shading, texture, binocular disparity). This phenomenon can be explained by the operation of a priori constraints in the visual system. In the case of shapes of polyhedra, the visual system minimizes the departure of contours from planarity and the variance of interior angles. As a result, when a slanted picture of a cube is rotated, the observer perceives a non-rigid cuboid, rather than a rigid picture of a cube.
An MPEG video illustrating this illusion is also available. Click here to get it.