This visual illusion (Van Lier, De Wit, & Koning, 2006) was also presented at ECVP 2005 (La Coruna, Spain -- it was selected as one of the best 10 static illusions and was honored with an exposition in the science musum of La Coruna).

Figures 1 and 2 comprise a grid of grey bars. The crossings of the bars have been ‘obscured’ in different ways.  In Figure 1, four three-quarter disks or ‘pacmen’ are slightly displaced with respect the outer boundaries of the crossing. Such pacmen configuration are known to induce the perception of an illusory square (Kanizsa, 1979, Organization in Vision, NY: Praeger). What is striking is the quite confusing appearance of the display. Observers may experience a percept in which the crossing of the grey bars is being watched through “pieces of glass”. Occasionally, one may also experience a displacement of the part of the grid that falls inside the illusory squares -- as if it has been clipped and shifted. This volatile percept seems related to the phenomenon in which small texture-like elements are captured by surrounding illusory contours (e.g., Ramachandran, 1985, Perception, 14, 127-134), although the present percept is quite unstable. It may further reflect the visual system’s attempt to deal with the perceived misalignment by momentarily (con-)fusing physical and illusory contours. Experiments confirm the perceived displacements in the pacmen configurations.

In Figure 2, a clear figure ground segregation can be seen -- very different from Figure 1. Here, the pacmen have been replaced by crosses (at the same positions). As these cross configurations do not elicit illusory surfaces, the ‘restlessness’ of the display has disappeared. 
 

Figure 1. Pacmen configurations are misaligned with an underlying grid. The percept is quite ‘restless’. 
  
  
 

Figure 2. Cross configurations that are misaligned with an underlying grid. The ‘restlessness’ has now disappeared.