This is the kind of situation that is not prescribed in the law and that will depend on the analysis of the situation by the judge assigned to this case. Either way, you will be in deep shit.

For most crimes, in many jurisdictions, there's a required element of mens rea: if you didn't have any intent to commit a crime, nor negligently or recklessly allowed it to happen, you're not guilty.

At least the 4th bullet is not how security works.

@NateEldredge PyRules, It might be reckless of one to bring that on the plane. What kind of cold thing would it have to come into contact with to become a sword? Can you give an example?

@Fiksdal I believe it just needs to be cold enough to freeze water. Liquid nitrogen would do it quickly.

I don't think anyone would bring dry ice or liquid nitrogen onto a plane, so that leaves us with rocky road, which is basically ice cream, right? So if you have the sword mold on your lap and someone is eating ice cream drops it on your sword, is that going to make it become sword? I'm trying to think of a scenario in which the mold could become a sword.

@Fiksdal maybe they have a cooler and it accidentally gets in it. Or the crew set the climate controls wrong in the cargo area.

You mean the cargo area where all the suitcases are? The one that's completely sealed off from the passengers? If so, there's no problem. It's legal to check in a real sword there.

Or do you mean the place for hand luggage, above the seats? That place wouldn't have a sepreate climate control. If that was below freezing point, the whole plane would be. And if that happened, an icy sword would be the least of the crew's problems.

How cold does the item the sword comes into contact with have to be? Airplanes are actually not cooled, they are actually heated. The outside air is extremely cool, thus the need for heating the cabin. The reason for the coolness inside planes is because they don't heat it more than necessary.

@Fiksdal It's about the same melting point as ice.

Is a small contact with ice cream enough to make it become a sword? Like someone dropping ice cream on it?

Ok... Well ice cream doesn't really vary a lot in temperature. So it sounds like accidentally freezing this sword on the plane is implausible in the first place. It would have had to be done on purpose, like if he also brought something to cool it with. If normal ice cream can actually freeze the sword. Then it would be careless to keep the sword mold up when somebody is eating ice cream. But taking the sword mold onto the plane wouldn't really be careless, as it's extremely unlikely to actually freeze on the plane.

@Fiksdal is it illegal to cause weapons to exist on a plane?

@PyRulez maybe not, but if you did it intentionally or negligently t a judge could probably just use the law that says that it's illegal to bring them on, arguing that you brought the weapon on board.

I would suggest a less implausible example, such as a piece of glass (bottle, framed picture) breaking.

I have wondered about something like this myself, although simply about guns. If you take them apart, at some point they are just pieces of metal and it's not like it's illegal to carry metal.

@R.Schmitz In Denmark it is illegal to carry gunparts (except if you have a license for the one particular gun it belongs to). Unless you deform the parts so that it's clearly not a gun part (basically, melting them down), I (a total layman) suspect you'd get into trouble for carrying them.

@R.Schmitz, the solution in the US was to designate one specific part (usually the receiver or frame) as constituting "essence of gun": if you've got that part, you've got a gun; otherwise, you've just got a bunch of metal bits.

Another less implausible example is what led to the liquids and gels rule. Lots of chemicals are harmless in isolation but can be combined in a way that gives rise to a chemical reaction that produces something dangerous. These could combine intentionally or unintentionally because containers leak or break.