I put my Fender Blacktop back together today, and left out the shim that was in there when I took it apart. It's just half a business card, not folded or anything. Anyway, when I put a string on it, it was laying flat on the fretboard at the heel. I took the neck off and put the shim back in, and now the action is crazy high. I never realized a tiny shim could make that big of a difference.