Has there ever been a case of an unfair treaty occur anytime through history due to use of biased map projections, either accidentally or deliberately?
One example would seem to be the 1867 treaty by which the United States purchased Alaska from Russia. The issues that arose are described Michael Byers and James Baker by in their book International Law and the Arctic.
The border between the two countries was to be in the Bering Sea and Bering Strait. However, as Byers and Baker observe:
... the 1867 Convention was silent on the type of line, map projection, and horizontal datum to be used in depicting the boundary. The two countries, which took different approaches to mapping, were consequently unable to agree on the precise location of the line.
Vlad Kaczynski, quoted by Byers & Baker elaborated:
Cartographers normally use two types of lines to delineate marine boundaries. These are rhomb [rhumb] lines and geodetic lines (also known as great circle arcs) that are used on two common map projections, Mercator and conical. Depending on the type of line and map projection used, lines will either appear as straight or curved lines. For example, a rhomb line will be a straight line on a Mercator projection, whereas a geodetic line is curved. Because each country interpreted the line described in the 1867 Treaty as a straight line, the Soviet Union depicted the Bering Sea marine boundary as a rhomb line on a Mercator projection whereas the US used a geodetic line on a conical projection. While both appear as straight lines on their respective map projections, each country’s claim maximized the amount of ocean area and seafloor under their respective control.
- Michael Byers & James Baker, International Law and the Arctic, Cambridge University Press, 2013, p33
This led to a disputed zone of approximately 15,000 square nautical miles. Each side, presumably felt the other's claims were 'unfair'. The dispute wasn't resolved until the Bering Sea Treaty was signed in 1990, some 123 years after the original treaty was signed.