While the casing on the right in the photograph shows a slightly different firing-pin imprint and may indeed have been the culprit (Mike didn't specify which one it was), that is not what I would consider to be a light primer strike. Rather, it appears to me to be a clear stike pattern of sufficient depth to have ignited a properly functioning primer. The slight difference in appearance may be due to a slightly harder primer -- but having been struck as hard as it was to leave the resulting impression, the primer material should have ignited and the cartridge should have fired.
"S***" happens, as we used to say, and even Speer and its superior Gold Dot ammunition are not exempt from "Murphy's Law." I think this was simply an extremely rare Gold Dot "dud" caused, as previously speculated, by either a glitch during manufacturing and/or contamination of the shock-sensitive explosive primer material by a penetrating agent of some type (e.g., solvent, oil, moisture, etc., etc.).
