Michael Barany at JSTOR Daily:
In 1935, one of France’s leading mathematicians, Élie Cartan, received a letter of introduction to Nicolas Bourbaki, along with an article submitted on Bourbaki’s behalf for publication in the journal Comptes rendus de l’Académie des Sciences (Proceedings of the French Academy of Sciences). The letter, written by fellow mathematician André Weil, described Bourbaki as a reclusive author passing his days playing cards in the Paris suburb of Clichy, without any pretense of overturning the foundations of all of mathematics (that more disruptive part of Bourbaki’s oeuvre and ambitions would come later). On the strength of Weil’s recommendation, Cartan helped launch what would become one of the most storied and notorious careers in the history of mathematics.
Weil, for his part, was playing a prank, propagating an elaborate in-joke that continued among mathematicians for decades. The math was real. Nicolas Bourbaki was not.