The break of the curveball « Best Visual Illusion of the Year Contest
http://illusioncontest.neuralcorrelate.com/2009/the-break-of-the-curveball/

Illustration of why curveballs seem to suddenly drop.

Optical illusion

Our illusions suggest that the perceived “break” may be caused by the transition from the central visual system to the peripheral visual system. Like a curveball, the spinning disks in the illusions appear to abruptly change direction when an observer switches from foveal to peripheral viewing.