The following code, depending on what angle you view the sphere from, will
sometimes display hidden faces entirely, and sometimes show them properly
shaded by the semi-transparent fill on the sphere.
void setup() {
size(400,400,P3D);
}
void draw() {
background(255);
translate(width/2,height/2,0);
rotateX(mouseY*TWO_PI/height+HALF_PI);
rotateY(mouseX*TWO_PI/width+HALF_PI);
sphereDetail(20);
fill(100,200,50,200);
sphere(100);
}

second item under "common issues / not bugs" in the faq:
http://processing.org/faq/bugs.html#issues

*** This bug has been marked as a duplicate of 176 ***