Do you think when he reseated the CPU cooler, the backplate could have adjusted to make contact with some capacitor leads or any such metal. Ive heard of people with this situation and looking on Tom's hardware I see similar situations to ops where they had reseated the CPU ect and it lead to dimm failure. In their cases they used electrical tape to insulate and reported success but with this situation I doubt the optiplex has a removable backplate