A handful of stamp forgers have produced impressive fakes.
Sperati was among the best at it, often using cheap genuine stamps and removing part or all of the design to replace it with elements from rare stamps.
He often sold them as reproductions or "philatelic art"
One group got him arrested not for forgery, but for exporting valuable stamps without a license. They fooled both Customs and their expert, until he produced an identical group. He was aquitted as his defense was that the stamps intercepted by customs were all forgeries!
https://www.sperati.org/Technique.htm
Someone like that turned loose on cards....