MLB pitcher R.A. Dickey* slings an erratic knuckleball pitch, posing a challenge for batter and catcher alike. The ball has been colorized to highlight its almost total lack of spin, which usually serves to stabilize the ball's trajectory.
A baseball's seams make it an imperfect sphere, which can lead to some screwy flight behavior when there's no spin to steady things out. Ivan R. Dee provides a good explanation of knuckleball dynamics in chapter 1 of The Knucklebook:
In the action of the knuckleball... some stitches are moving toward the flow of air in the front, and others are moving away, at a slow speed. The stitches move around the ball in quire a complex curve on a knuckleball, and the ball may rotate at different rates in different ways. This causes the swirls of air to change size and direction, form and disappear, and move location on the ball, thus producing changing locations of low pressure that really can't be predicted. The wake behind a single knuckleball at various points in flight may look like this: