A postage stamp error is any of several types of failure in the stamp printing process that results in stamps not having their intended appearance. Errors include use of the wrong colors, wrong denominations, missing parts of the design, misplaced or inverted design elements and more. The term “error” is typically reserved for obvious failures in the production process that (potentially) replicate over many stamps; unique errors or poor quality stamps are known as “freaks” or “oddities”. Printing plate flaws, such as cracks, wear, or even constant flaws and plate repairs are not considered errors.
Genuine errors are uncommon or even rare; postal administrations have several layers of quality control and inspection. Most printing problems are addressed before the stamps ever reach the public. A particular error may only exist in a few dozen copies, and some well-known errors, such as the Treskilling Yellow, are unique (so far as we know). They are prized by collectors, with some fetching prices thousands of times higher than the normal stamp of their type.Errors are known to occur at any stage of production, starting from design to engraving, to replication of the dye, to printing itself, and to perforation. (In theory gumming errors are possible. In practice, used stamps have no gum, so any error would become undetectable).
Following is a list of the major types of stamp errors:
Design error: The picture may be the wrong subject, maps may show inaccurate borders, the inscription may be factually wrong, text may be misspelled, etc.
Value error or substituted subject: A die composed of multiple elements has the wrong elements mixed together, for instance a low-value denomination being used on a design intended only for high values.
Omission error: Part of the stamp’s design is missing.
Missing color: Associated with printing processes that print the stamp in several different colors.
Missing overprint: A stamp valid for postage only when overprinted but with a missing overprint.
Double impression: Stamp, or overprint, was printed twice, one impression offset from the other.
Invert error: Part of the stamp is printed upside-down.
Inverted overprint: The overprint is printed upside-down.
Color error: Stamp is printed in the wrong color(s).
Paper error: Stamp is printed on the wrong type of paper which may have a different watermark or color than intended.
Imperforate error: Perforations are missing on one or several sides. As perforations may be removed by cutting them off, imperforate errors are collected in pairs.
In philately, “errors, freaks, and oddities” or “EFO” is a blanket term referring to all manner of things that can go wrong when producing postage stamps. It encompasses everything from major design errors to stamps that are just poorly printed. EFOs can include some of the most highly sought and expensive of all stamps, and others that attract the attention of only a few specialists.
Once the yellow-inverted error of the U.S. 1962 Dag Hammarskjöld memorial stamp was discovered, 40,270,000 were printed to prevent speculation. Only the original unintentionally printed specimens are considered to be errors.
An error is any sort of production mistake that is (potentially) replicated on many stamps; the famous Inverted Jenny is the best known of these. A sheet of partial prints was accidentally re-inserted into the printing press upside down for the second color, resulting in an invert error. Design errors can include wrong dates, wrong names, wrong pictures, anachronisms and the like. Color errors include stamps like the Treskilling Yellow which should have been green, as well as missing colors in modern multi-colored stamps. It is not especially rare for the perforating equipment to malfunction and result in imperforate errors.
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 something that is within the bounds of usability for the stamp, but still has a distinctive appearance. The usual sort of oddity is misregistration on a multi-colored stamp, which can result in shirts apparently with two sets of buttons, eyes above the top of a person’s head, and so forth. 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 some care to ensure that mistakes don’t get out of the printing plant; to be valid, the EFO stamps must have been sold to a customer. Mistakes smuggled out by unscrupulous 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.