Mechanical doubling, also called machine doubling or strike doubling, a form of doubling which appears on a coin and is easily confused with doubled die strikes. On a true doubled die coin, the doubling comes from a mistake in the process used to make the coin die itself. In mechanical, or machine doubling, the doubled image results from mechanical issues during the striking of the coin, such as the coin shifting during striking, or the die itself being jarred out of position as a result of vibration or improper coin press maintenance. Mechanical, machine, and strike doubling are all considered by purists to be forms of damage, and are therefore not collectible as an error coin.
See also: Doubled Dies and Die Abrasion Doubling
Most of the time, when novices think they have found a new type of doubled die, they've actually found a coin exhibiting worthless mechanical doubling instead.