the terrorist only has to find ONE thing that is a useful weapon but is NOT on the list
Quite so,
CJ Driver - in fact it's even worse than that. As no less a security guru than
Bruce Schneier has pointed out, there's no penalty (other than confiscation) for attempting to carry on board an item on the 'forbidden' list. So a bad person who has thought of a method of bringing down an aircraft with (e.g.) 150ml of apparently innocuous liquid can simply carry on making repeated attempts to get through the system. As the system can never achieve 100% effectiveness, he will eventually succeed. (We'll gloss over the point that two bad people could carry through 75ml each with no difficulty whatsoever.)