Look directly at the spinning disk and it will appear to fly straight. Look at the purple "fixation point" at the right, and the disk will keep sliding to the side. It is probably the eeriest thing you've seen today, and this paper will tell you why you haven't really seen it.
Like something coming to life that you can only see out of the corner of your eye, as soon as you shift your eyes away from the spinning disk, it zooms off its straight line and curves out to the side. This sudden jump is caused in part by the movement of your eyes. It's also caused by the fact that your peripheral vision isn't as good as your direct vision at picking up on motion cues. According to scientists, this is the reason a curveball in baseball seems to suddenly "break" and curve wildly to one side. They explain the illusion, stating:
The curveball's break is a perceptual illusion caused by (1) the inability of peripheral vision to maintain separate representations of different motion signals, and (2) gaze shifts during the curveball's flight. The discontinuity is therefore a result of the change in the neural response at the transitional moment when the image of the ball – or a portion of the image – is transferred from the fovea to the periphery (or vice versa).
The effect isn't always entirely an illusion. A correctly-thrown ball does really curve, but it does so slightly, and it does so throughout its flight. It doesn't suddenly leap sideways. If you want to know more about how your eyes will always deceive you, read the entire paper at PLOS One.