South Vietnam; 12. October 1966; Inauguration of the new World Health Organization headquarters in Geneva; spectacular block of 12 that shows a perforation error. The horizontal and vertical perforation is incomplete. This most likely happened as the sheet got stuck in the machine due to mechanical failure.
After it was released it jumped an inch leaving part of the sheet imperforate. Since all South Vietnamese stamps were produced overseas (mostly by de la Rue in Europe) errors very rarely occur. Product Control was simply better in Europe than in Vietnam. Very rare.