A rare 24 cent Inverted Jenny U.S. postage stamp sold at auction in New York in 2016 for $1.3 million. A hundred stamps depicting Curtiss Jenny biplane were accidentally printed upside down and circulated in 1918 until someone noticed the error, making them highly collectible.

Ripley's

I don't collect stamps, but I do collect Pokémon cards, so I guess I could understand the value. Also, what is a Curtiss Jenny biplane?