Stamps without perforations, with missing or incorrect color, missing or inverted writing on the reverse are all major errors. Smaller flaws, such as color shifts, misplaced perforations, hickeys (or doughnuts) and other such anomalies are termed freaks, rather than errors. These, too, are collectible and have value, but they do not command the same attention as major errors. Major errors are extremely rare and exist in small numbers. All errors and freaks on duck stamps are very desirable and add a great deal of interest and value to a collection.