Alan Nathan examines the flight of the knuckleball:

The smoothness conclusion appears to contradict the popular belief that knuckleball trajectories are erratic and often experience abrupt changes of direction. Let me speculate that this belief is the result of the randomness of movement, both in magnitude and direction, giving rise to the perception of erratic behavior. We have all seen instances where the catcher and pitcher get their signals crossed, and the catcher has to lunge for the ball at the last moment. The catcher expects a certain movement, and the pitcher throws something with different movement. With the knuckleball, no one really knows what movement to expect, so it is not surprising that the catcher has some difficulty cleanly catching the ball and that the batter has even more difficulty hitting it. There are other instances where claims based on perception have been shown to be unsupported by the data, such as “late break” and the “rising fastball.” I don’t doubt the perception, but I prefer to rely on scientific evidence when it comes to reality. With apologies to John Walsh, I conclude that knuckleballs are more like bullets than butterflies.

Hat tip, The Book Blog.