Errors, freaks, and oddities

In philately, errors, freaks, and oddities (EFO) collectively refer to the wide variety of mistakes that may occur during the production of postage stamps.^[1]

Postal authorities generally take some care to ensure that mistakes do not get out of the printing plant; to be valid, the EFO stamps must have been sold to a customer. Mistakes smuggled out by employees are called printer's waste, not recognized as legitimate stamps, and may be confiscated from collectors; the Nixon invert is a well-known recent example of an apparent new error that turned out to be simple theft by insiders. The authorities may attempt to lay hands on legitimately sold errors, as happened with the original Inverted Jenny sheet, but usually, collectors are smart enough to hang onto the windfall.