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Here's another nice example:
You may think you are looking at a spiral, but in fact, this is a picture of concentric circles (smaller rings placed within larger rings). Try following the outermost circle all the way around (you may find this to be quite a challenge).
As you do so, notice how your eyes tend to slip inward to the next circle. This illusion probably results from the unusual stripe pattern on the rings. Your eyes tend to want to follow a single color and treat it as an unbroken line. For example, if you follow only the orange parts, there will be a break in the line every time it turns black. That break is, perhaps, more noticeable than the break between concentric circles (especially near the top where the circles are close together).