Catching patches 
Van Lier & Vergeer (2006)

This visual illusion was selected as one of the Top 10 illusions in the 'Best illusion of the year' competition (Sarasota,  2006).

Below is a dynamic version of the illusion.

Note that grey patches arise at the crossings of two homogeneously colored outlined bars.

Here is another dynamic version.... 

The patches that appear at the crossings in the Hermann grid are known to disappear when the straight contours in the grid are distorted, e.g. by curvature (Geier, 2004). Here, we present an illusion based on Hermann-grid like gratings in which the contours are quite randomly distorted. These distortions guarantee a severe reduction or complete disappearance of the visibility of the patches. Starting with these gratings we show that the patches at the crossings return when luminance edges are introduced and extended at the intersections. The ‘returned’ patches have the same relative lightness properties as they would have in a regular Herman grid (dark patches when the crossing bands are relatively light, and light patches when the crossing bands are relatively dark). In addition, the polarity of the perceived lightness difference does not depend on the lightness of the edges (i.e., whether they are dark or light). A remarkable effect here is that at the crossings the whole area between the edges is perceived to have a different lightness, irrespective of the shape of that area (i.e., whether the edges bend inward or outward etc.). Measurements on perceived lightness differences confirmed the above observations (and also for observations on various other edge manipulations). A manuscript is currently in preparation (Van Lier & Vergeer). To illustrate this illusion, we have included various examples (also an example based on one of the Hermann grid manipulations of J. Geier). 

Here is a Powerpoint file with various examples of the illusion.