After pitching for 12 years, some of those games pitching the entire game, Koufax was forced to quit due to the pain in his arm. He was one of the best pitchers in MLB at the time. This changed history because more pitchers realized they had to be cautious in order to not throw their arm out too.
George Brett, after hitting a 2 run homerun against the yankees, was caught for having too much Pine Tar on his bat. He was called out and the Royals lost the game. This changed history because it alerted players that the rule was serious and they could get in alot of trouble for violation of that rule.