The blue elements in the image above appear to be a arranged in a continuous spiral, but in fact they form a series of concentric circles. Your brain will argue so strongly for a spiral that you may need to run your mouse cursor around the circles a few times to convince yourself.
Image credit to Akiyoshi Kitaoka 2009. Via Moillusions.
http://www.psy.ritsumei.ac.jp/~akitaoka/uzu7e.html