A simple bug in the embedded iteration example.
The height and width were swapped.  I've fixed this, and also added an 'if' statement to catch negative box sizes.
I've pasted the fixed code below:

float box_size = 11;
float box_space = 12;
int margin = 7;

size(200,200);
background(0);
noStroke();

//draw gray boxes

for(int i = margin; i < height-margin; i += box_space){
 if(box_size>0){
   for(int j = margin; j < width-margin; j+= box_space){
     fill(255-box_size*10);
     rect(j, i, box_size, box_size);
   }
   box_size = box_size - 0.6;
 }
}