Impossible Trapezoid Optical Illusion

What's wrong with this picture?

See Another Random Optical Illusion

Explanation

Notice how the shading works to create a believable yet impossible object. The use of different shades around the figure helps the eye track and believe this 3D optical illusion.

The illusion also uses perspective angles. The surfaces and corners are not in the same position they would be if this were a normal trapezoid, drawn in proper perspective.