The Poggendorff illusion is named after Johann Poggendorff who identified it in 1860. It is a geometrical-optical illusion which involves the misperception of the position of a segment of a transverse line, which has been interrupted by the contour of an intervening structure (a rectangle).
Source:
Howe, Catherine Q., Yang, Zhiyong, Purves, Dale (2005), The Poggendorff Illusion Explained by Natural Scene Geometry (PDF).