Stamps with major, consistent, and unintentional deviation from the normal stamps are considered as errors. These errors are caused rarely during the stamp printing process. Error stamps do not show the intended appearance of the desired stamp. There are some factors that cause postage stamp errors like wrong denominations, wrong or missing colors, misplaced or an inverted design element, wrong stamp paper, wrong watermark, double impression, etc.
The stamps with poor quality are known as freaks and oddities. Stamp errors, freaks, and oddities are terms referring to all kinds of things that go as an error when producing postage stamps. Sometimes mistake in the error stamp may be replicated with other stamps accidentally. Freak includes paper folds such as half-printed half-blank stamps, perfs running diagonally across the stamps, etc. Some factors that cause freak stamps are the complete absence of colors, misalignment of stamp sheets, too little or too much ink printed, corners of the sheet is folded over or under and others.
Oddities are the stamps with a distinctive appearance. Stamp errors are uncommon or even rare. There are various layers of quality control and inspection done by administrations. Usually the printing problems are informed to the employees before the stamps reach the public. Stamp gumming errors are due to imperforation, lack of separation of press stamp sheets, too big or too small perfs, and stamp is gummed on the wrong side or on both sides.
Stamp design errors are those stamps with wrong subject, maps showing wrong border, wrong stamp inscription and stamp texts are misspelled. These are the errors that are made during the design phase of the stamp production process. Omission error stamp is the major types of error stamps. They occur when part of the stamp design is missing. An invert error stamp occurs when part of a stamp is printed upside down.