Not all holes are grounding points. Often times, the one or two holes in the very corners are not grounding points as they are not standard ATX formfactor mounting holes. You can determine whether a mounting hole is a ground by whether or not the tin/solder/metal ring around the holes is present or not. Holes without metal rings around the edges are not grounds and should NOT come into contact with metal stand-offs; insulating material should be used. It's rather unlikely that using the metal stand-off for this corner hole is going to short anything out, but the possibility exists.

Only use metal stand-offs for holes that have metal rings around them and thus are grounds. In that case, it is important to use proper metal stand-offs that will provide a ground for the motherboard.