Named after Johann Poggendorf who first described the effect in 1860. The setup for this illusion is a straight line that passes behind a rectangle. This effect will test your ability to map a straight line across an obstruction.
Try the demonstration. Drag the top line segment back and forth until it looks like it is lined up perfectly with the bottom portion. Now click the Show button. How far off were you? This is a test you can’t fail. We all have trouble with this illusion to some extent. Reset will automatically align the two line segments.