A point that hasn't been covered is the problem of language misunderstanding. Yes, it's stupid to joke, but let's suppose we have a security officer with a broad accent questioning someone whose command of English (or whatever the local language) is dubious. A classic example of this was a few years back where a German on a plane from Miami (IIR American Airlines) was bursting for a pee. The seatbelt sign was on and he used the German expression 'I am exploding'- because his English wasn't very good... The FA thought he was making a bomb threat, the flight returned and he was in jail for some time. IIR, the judge threw the case out.....
Now consider the passenger travelling with professional equipment - say a bomb calorimeter, or even someone taking an ice cream bombe - correct terms, but not necessarily in everyday English. Declaration of these in checked baggage, especailly by a non native English speaker could well lead to all sorts of problems, so in the end security people need a lot of training in the sort of things they need to meet. Could easily lead to them thinking something is a threat or joke when it isn't.
The problem of hypodermics is another one that can't be readily answered. Or do you tell people needing injections that they can't fly?
BTW, are there still scissors in the on board first aid kits?