The issue is shadow coming out the side of the containing div. In order to avoid this, the blur value must equal the absolute value of the spread value.
div {
-webkit-box-shadow: 0 4px 6px -6px #222;
-moz-box-shadow: 0 4px 6px -6px #222;
box-shadow: 0 4px 6px -6px #222;
}<div>wefwefwef</div>covered in depth here