You received a border warning with your Online PDF proof if your design contains any kind of border.
Please note that any border along the edge may make your artwork appear uneven or off-centre on a portion of your order. This is due to slight shifts that may occur during the printing and trimming process (which are unavoidable for any printer) - with a border running parallel to the edge, any shift (even the slightest) becomes more noticeable.
Business Card Potential Shifts