Argentinian stamp claiming Falkland Islands, South Georgia, and South Sandwich Islands as well as some of the Beagle Channel Islands and Cape Horn, which are administered by Chile.
Irish stamp (first issued 1922), which omits the borders of Northern Ireland.
Stamp of the Dominican Republic which shows that country's borders extending into Haiti, and which caused a boundary dispute to occur.
And then there is the case of the Gran Chaco War, fought between Bolivia and Paraguay.
The Gran Chaco is an alluvial plain covered with grass and hardwood forest, located in the area where Argentina, Paraguay, and Bolivia's borders meet. It was formerly thought to be rich in oil, and was Bolivia's only access to the Atlantic Ocean.
Paraguay laid claim to the area, and in 1924 issued a stamp series which implied that the area belonged to them. Note that the first six letters of the country's name are spread over this area:
Bolivian complaints forced withdrawal of the stamps, but three years later, Paraguay issued another set of stamps showing the area squarely within Paraguay's borders:
In 1930, Bolivia issued stamps showing it within Bolivian borders:
The sabre-rattling continued when Paraguay issued a stamp in 1932, identifying the area as "Chaco Paraguayo" with the inflammatory slogan underneath "Ha Sido Es Sera", meaning "Has been, is and will be":
Shortly after this, war began between both countries over the area, which lasted until 1935, in which an estimated 80-100,000 died. Paraguay, despite a smaller population and smaller army prevailed for various reasons. The Treaty of Buenos Aires of 1935 ended the war, awarding the area to Paraguay, but also allowing Bolivia navigational rights to rivers in the area.
A final treaty clearly marking the boundary between the countries was not signed until this past April, on April 28, 2009.