Errors stamps have mistakes in color, perforation, or design. Freaks show an inconsistency in their production. Some collectors group all such material into one category called errors, freaks, and oddities, or “EFO’s.”
The term “error” generally refers to production errors, as opposed to design errors. Design errors occur when a stamp’s topic is based upon incorrect information. An example is the 1994 Bill Pickett stamp in the Legends of the West series, which actually portrayed Pickett’s brother Ben.
Inverts are stamps with a portion of the design printed upside down. Inverts must have at least two colors and require more than one “run” through the printing press. Most inverts are created by placing a sheet incorrectly into the printing press – as with the 24-cent Jenny Invert, where a sheet of stamps was loaded in reverse between the printing of the red frame and the blue vignette. Inverts are the most famous of all errors, and the “Jenny” is the most famous invert!
Imperforate errors occur when contrary to design specifications, no perforations exist between two stamps. These stamps are not to be confused with those purposely made without perforations. In the case of imperforate errors, if there is one perforation hole, or even an indentation created by a perforation pin (the device which punches the hole into the stamp paper), then the stamp is not a true error. Imperforate errors are usually collected in pairs or larger groups, as it is extremely difficult to tell a true imperforate error from a trimmed stamp.
Color-omitted error stamps have one or more colors missing. These errors usually occur when one stage of a multi-run printing process is skipped. Since older stamps involved fewer colors in their printing, this type of error occurred less often as they were easier to spot. Today’s multi-colored issues are much more complicated (concerning color) and errors are much harder to identify. This explains the recent increase in color errors.
Freaks are often caused by mechanical means, not human error. In addition to being caused by a malfunctioning machine, they’re often the result of foreign material on a printing surface. In short, freaks are created by what can be seen as a “fluke” in the stamp-creation process. Various kinds of freaks include mis-perforations, foldovers, mis-cut booklet panes, color mis-registration, and a few other minor variations.
When a stamp’s perforations fall within its design rather than in the margin, then it’s misperforated. Sometimes the perforation goes directly through the center of the stamp. Foldovers happen when a part of the stamp sheet being printed, usually the corner, becomes turned. It causes that part of the stamp’s design to be printed on the reverse side of the sheet or the perforations to go off at strange angles, creating unique variations. Out-of-register color impressions can have quite a variety of effects on the appearance of a stamp. Sometimes the result is an off-center portion of the stamp. Other times, out-of-register printing creates a dramatic double-image.