Tête-bêche

"head-to-head") is a joined pair of stamps in which one is upside-down in relation to the other,[1] produced intentionally or accidentally.

Mechanical errors during the process of production can result in tête-bêches.

Where the columns of stamps from each side meet there is therefore a row of tete-beche pars.

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.

Tête-bêche pairs of the Swiss "William Tell's son" design of 1910 are relatively common.
Tête-bêche layout required that the 1921 overprints of Switzerland were arranged to match.