The 100 bills that scared the government into currency changes came in about twenty years ago. They were so good the federal reserve couldn’t detect them. All had different serial numbers and they were printed on the same rag paper we use.
A guy busted for drugs blew the whistle. They didn’t believe the money was counterfeit when he told them.
During WWII the Germans got Jews out of concentration camps who were master engravers. They produced perfect pound notes but the British were still fairly successful using serial numbers and determining to which banks etc, the particular notes went.
The Germans Abwehr paid their spies in counterfeit bills which may not have been smart.