This isn't really an optical illusion, though - the vertical lines are really arranged in the shape of circles. I find it interesting that you can't mentally "tune out" the horizontal lines!
Okay...I put a bit of work into taking a screenshot and in the following image I smudged the bottom edges of some of the circles. In a given row, 3 circles got smudged to reveal the bottom edge but left the 4th circles untouched: