don't use this old stamp to send a check for a magazine subscription

The Inverted Jenny (1918), a misprinted USA stamp believed to be one of the most rare and valuable of all postage stamps.

"... It is believed that only eighty of these stamps exist today. It remains a mystery as to what happened to the remaining 20 stamps. A single Inverted Jenny stamp may sell for $100,000 or more, depending on its condition. Only seven of four-stamp blocks exist and each is worth $1 million or more."