USA $100,000 (1934)

It's presumably illegal for the same reason 1933 double-eagles are illegal: if all 42,000 notes have been "accounted for" and none of those official accountings are in private hands, then logically any that turn up in private hands must have been stolen.

They used to have one on display in the Smithsonian US History museum. They took down the entire US money display a few years ago and they were going to redistribute the items throughout the museum, so hopefully it is being displayed somewhere and not locked up in some dungeon.

There is a sheet of them on display in the library of the ANA Museum.

There is nothing contradictory or confusing about it at all. Simply put, $100,000 Gold Certificates were only used for transfers between the Federal Reserve Banks. They were never used by public banks nor were any ever released for use in commerce. Since these notes represented a massive amount of money at the time they were created, their use was thoroughly accounted for and tracked via the serial numbers. When they were withdrawn from use by the Fed, most were destroyed and the rest became museum pieces. Since every note is accounted for and none were ever circulated, anyone claiming to own one is only holding a fake piece of paper. The illegal possession is a bit of a red herring since a note would have to be initially stolen anyway to make it into private hands.