My favourite story right now.
A young man named Adolf "Adi" Dassler started making shoes in his mother's house in 1920s Herzogenaurach (Germany). His brother Rudolf joined forces with him and they formed the Dassler Brothers Shoe Factory. They became successful and gained international exposure during the 1936 Berlin Olympics hosted by Nazi Germany.
In the aftermath of WWII, however, Adolf and Rudolf had a major disagreement. This was partly because their respective wives did not get along, and partly because Rudolf had been forced into conscription and accused his brother of being responsible. The situation escalated to such an extent that they split the company into two.
Adi Dassler named his new shoe company after an abbreviation of his name, and Adidas was born. Rudolf's company was initially named Ruda, but was soon changed to Puma. Today, Adidas is the world's second-biggest sportswear brand (trailing the American Nike brand) and Puma is number three.
Because Herzogenaurach is a small Bavarian town with the two global companies completely dominating local employment, Adolf and Rudolf's animosity divided the entire community. A fierce rivalry arose between two local football teams, each sponsored by the respective brands. Employees of Adidas and Puma avoided speaking to one another. Herzogenaurach was at one point nicknamed the 'town of bent necks', as people would look at each other's shoes to see which brand they were affiliated with.
Both Adolf and Rudolf passed away during the 1970s, and were buried at opposite ends of the Herzogenaurach cemetery. Only in more recent times has the rift in the town healed somewhat.