Did Capacitative Loading Kill My Op Amp?
By the sound of it, this is what went wrong... 

The leads that feed power to the prototype have inductance and store a few tens of nano joules of energy when taking 0.5 amps. If the current is open-circuited rapidly (as per disconnecting your ammeter) this energy is released onto the circuit and without a decent power rail decoupling capacitor, this can cause a voltage spike of tens to hundreds of volts superimposed on the 12V rail. This would instantly kill the op-amp and in those short microseconds could cause the op-amp to destroy the FET by applying to large a voltage to the gate.

You need a supply decoupling capacitor and 1nF would be just about enough to cope but, it's fairly standard practice for at least 10nF to be applied if not 100nF.