Café wall illusion
When you look at this photo, the horizontal lines appear to be tilted or sloped. What is happening is that your brain is trying to make sense of the lines, so it focuses on the blocks. The blocks are offset, so the shift in the blocks tricks your brain into seeing slanted layers. In reality, the lines are perfectly straight, it's just your brain playing tricks on you.
Book: DiSpezio, Michael A. Optical Illusion Magic: Visual Tricks & Amusements. New York: Sterling Pub., 1999. Print.
Photo: "BrainDen.com - Line Illusions." BrainDen.com. N.p., n.d. Web. 18 Mar. 2015.