Any sufficiently advanced technology is indistinguishable from magic. (Arthur C. Clarke's Third Law of Prediction)
Any technology which can be distinguished from magic is insufficiently advanced. (Gehm's Corollary to Clarke's Law)
There is no technology, no matter how advanced, that cannot be circumvented by a sufficiently determined individual or group. (Janet Miles, to the best of my knowledge)
No matter how subtle and powerful the wizard, a knife between the shoulderblades will seriously cramp his style. (Steven KZ Brust)