The Poggendorff illusion is very strong at first for most people. They believe the top line is too far to the right when they are actually lined up. However, if you look at the illusion many times, you gradually stop being fooled by it. Some people never see the illusion at all while very young children see the illusion most readily.
There are several ways of trying to explain how this geometrical illusion might work. The most common explanation is that we misjudge the angle at which the line intersects the rectangle. When we look at the figure, our brain increases the angle between the line and the rectangle, trying to make it closer to a right angle. By increasing the angle, the brain emphasises the difference between the angle of the rectangle's side and the angle of the line. The brain often over-emphasises differences.