Wikipedia
In philately, tête-bêche ( French for "head-to-tail", lit. "head-to-head") is a joined pair of stamps in which one is upside-down in relation to the other, produced intentionally or accidentally. Like any pair of stamps, a pair of tête-bêches can be a vertical or a horizontal pair. In the case of a pair of triangular stamps, they cannot help but be linked "head-to-tail".
Mechanical errors during the process of production can result in tête-bêches. During the printing of stamps for booklets, the pages of stamps are usually printed in multiples from a larger printing plate. This can result in tête-bêche pairs. It is unusual for these pairs to find their way into the postal system, as they are cut into individual booklet pages before binding into the distributed booklet. A block of 24 5d Machin stamps, which should have been guillotined into four booklet pages, includes four tête-bêche pairs. This was sold in 1970, in the normal course of business, by the British Post Office and is exhibited by a member of the Royal Mail Stamp Advisory Committee.