Bruce Schneier has some interesting ideas on a pretty large logic gap in our airport screening processes.
He posits that the TSA is effectively looking for two distinct classes of contraband: the kind that will get you in trouble if you try to bring it on a plane (knives, bombs, guns) and the kind that they will happily take away from you and throw in the trash as you stand there (bottles of liquid, scissors, corkscrews).
Real terrorists probably won't try to smuggle in a gun or bomb via TSA screeners, because getting caught would put a crimp in their plan - security doesn't need to catch EVERY gun or knife, because just the threat of being caught is a good deterrent.
For the other class of contraband though, there is no penalty - if I'm trying to sneak liquid explosives or something in my water or shampoo bottle and they find it, they'll just toss it away and I can try again, and again, and again until I get it through. Now, security needs to find it 100% of the time to be considered effective.
I hadn't really thought of it from that perspective, but I am now.