United States stamp collectors can choose from an embarrassment of riches published over the past 150 years. Some of the most interesting and quirky United States stamp collections involve what are known as EFOs, or ''errors, freaks, and oddities.'' EFOs can be anything from offset prints to miscolored stamps to sheets missing coil numbers.

Of course, not all EFO stamps are particularly valuable. In 1962, the United States Postal Service printed a stamp honoring Dag Hammarskjold. This issuance somehow got miscolored, but by the time the error was discovered by postal officials, the stamp had gone out in mass quantities. That being said, some EFOs are extremely rare. The most famous rare EFO is a stamp printed in 1918 called the Inverted Jenny, sometimes referred to as the Jenny Invert.

The Jenny stamp features an upside down double-decker biplane, called the Curtis Jenny. Fully colored in red, white, and blue, and sold for a premium at 24 cents, the original Jenny stamp was designed to herald the advent of U.S. Postal Airmail Service. There was a huge rush to print out as many of these stamps as possible in preparation for the rollout.

One of the engraving machines accidentally got inverted, and 100 stamps went out with the double-decker biplane upside down. This Inverted Jenny stamp, which now sells at auction for $150,000 in mint condition, has become perhaps the most recognizable icon of the United States stamp collecting hobby. EFO collectors also like to go after machine-created ''crazy perfs,'' which are diagonally (or otherwise skewed) perforated stamps.