There's a bug in the Subtract layer effect (possibly other ones, too?) where objects in the layer "pop out" after increasing their alpha-blending levels.
Create a plain layer;
Create a second layer and give it the Subtract effect;
Place a screen-sized, solid-color Active Object in the Subtract layer;
Always add one to the object's Alt. Value A and set its alpha-blending to its Alt Value A.
You'll see that it suddenly pops between 230-255.
Here's an example: Please login to see this link.