A postage stamp design error is a mistake made during the design phase of the postage stamp production process. Design errors most commonly occur as minor mistakes, such as a missing letter in the binomial name of an organism depicted on the stamp, but some have been major gaffes, such as a map appearing to lay claim to another country's territory, or the depiction of the wrong person on the stamp.
A design error caught during the production process may disappear quietly, with copies of the error only getting into the public's hands via unscrupulous employees (these are therefore not considered "real" stamps). Design errors are often caught during the distribution process, when large numbers of postal workers are scrutinizing the new stamp; although officials may elect to withdraw all the stamps at that point, it is very difficult to retrieve every one of them, and in these instances a few may end up being sold and used. The exact circumstance are important, because once the stamp is sold to a customer, whether or not against the postal service's rules, it is considered to be legitimate.
Somewhat rarer is a design error that is first noticed by a member of the public. This usually happens within a few days of the stamp first going on sale, usually ends up as the subject of newspaper articles, and has been known to cause a diplomatic breach. The response of postal officials may include withdrawal of all the stamps, or simply the suspension of printing and distribution, pending revision and reprinting. If the stamps are withdrawn, then the ones already out there become instant rarities, as happened with the PRC's "All China is Red" stamp of 1968. The withdrawn stamps may be destroyed or overprinted if the design can be repaired that way.
Above design error on Philippine first day cover showing the wrong date. The date around the image should have been 1551, not 1951 (400 years of Antipolo). The postmark showed the right dates.