In philately, “errors, freaks, and oddities” or “EFOs” is a term that refers to all the kinds of things that can go wrong when producing postage stamps. It includes everything from major design errors to stamps that are just poorly printed; it includes some of the most sought-after and expensive of all stamps, and others that attract the attention of only a few specialists.
An error is any sort of production mistake that is repeated on many stamps; the famous Inverted Jenny is the best known of these and now extremely valuable, resulting from a sheet of partial prints being accidentally reinserted into the printing press upside down for the second color. The result was an invert error. It is also not especially rare for perforating equipment to malfunction and result in in perforation error stamps.
Design errors include wrong dates, wrong names, wrong pictures, anachronisms, and similar mistakes. A notorious example is the 2011 Liberty stamp error which shows the Statue of Liberty replica from the Las Vegas Strip rather than the original lady in New York harbor. Three billion of them were printed and sold. Color errors include stamps like the Treskilling Yellow which was meant to be green. Another well-known type of color error occurs when modern multi-colored stamps are printed with one or more colors missing.
Monaco stamp: The man’s left hand has five fingers and a thumb. Hubert Humphrey stamp: correct dates of his service are 1964 – 1968. Errored stamp show dates of service as 1965 – 1969.
A freak is a one-time mishap in the production process. Freaks include paper folds resulting in half-printed half-blank stamps, “crazy perfs” running diagonally across stamps, and insects embedded in stamps, underneath the ink.
An oddity is a stamp that can be properly used for postage but still has a distinctive appearance. The typical sort of oddity is misregistration on a multi-colored stamp. This can result in shirts apparently with two sets of buttons, eyes above the top of a person’s head, “moving” items and similar situations. These can be extremely common. The Canadian Christmas stamp of 1898, depicting a map of the world with British possessions in red, is famous for unusual color oddities that appear to claim all of Europe or the United States or central Asia for Britain.
Postal authorities generally take care to ensure that mistakes don’t leave the printing plant; to be considered an error, freak or oddity, the EFO stamps must have been sold to a customer in a post office or by the postal department. Mistakes smuggled out by unscrupulous employees are called printer’s waste, not recognized as legitimate stamps, and may be confiscated from collectors. Postal authorities may attempt to confiscate legitimately-sold errors, as happened with the original Inverted Jenny sheet, but usually collectors are smart enough to hang onto the windfall.