Glad to hear that your 9600GT was the only casualty.
I'm anal when it comes to unused psu connectors, including unused sata power connectors and fan connectors.
There are just too many ways that an unused connector can cause an electrical short unless it is properly secured so that it can't accidentally cause a short circuit. Proper cable management can help reduce the risk.
Even with a multi 12 volt rail psu, it's possible that the OCP (over current protection) can allow 25 amps or more current to flow.
My made by Seasonic, 750 watt Antec TP-750 TruePower New power supply has a 40 amp OCP setting for each of the four 12 volt rails. I'm positively shocked to hear about the premature demise of your 9600GT. May it R.I.P.