Mueller-Lyer illusion
The Mueller-Lyer illusion is a famous distortion optical illusion consisting of nothing more than an arrow. When viewers are asked to place a mark on the figure at the mid-point of the horizontal line, they invariably place it more towards the "tail" end (the real midpoint is marked in the diagram).
Another variation consists of two arrow-like figures, one with both ends pointing in, and the other with both ends pointing out. When asked to judge the length of the two lines (which are in fact equal), viewers will typically claim that the inward pointing pair is longer.