Zollner Illusion
The Zollner Illusion is a classic optical illusion that uses parallel lines and angles in ways that fool the eye and the brain to believe that the lines are not actually parallel. You can see an example here (leaving AlleyDog.com): http://psychology.about.com/od/sensationandperception/ig/Optical-Illusions/Zollner-Illusion.htm
See also: Mueller-Lyer Illusion