I believe the answer is: Müller-Lyer Illusion
The Müller-Lyer illuison is consist of a set of three arrow-like figures, where the head of the second arrows would be drawn on the opposite direction than the first.
When the participants are asked to determine which arrows are longer after discounting the arrow-head, most people see the arrow with backwards head placement has the longer line even though the length of all arrow lines are the same.