Here is the famous Cafe Wall illusion. This effect is another mind altering trip. The lines are all parallel to each other and straight as an arrow.
The background below is from the Exploratorium in San Francisco, CA. They have an outstanding site start with thier animation of this illusion so you can better understand the effect.
The Cafe Wall Illusion was first described by Dr. Richard Gregory. Dr. Gregory is a good friend of the Exploratorium and director of The Exploratory Hands On Science Museum in Bristol, England. Dr. Gregory observed this curious effect in a pattern of tiles on a cafe wall in Bristol.... Note that the effect is not visible in the picture due to the small scale of the tiles.