Curse of the Bambino
The Boston Red Sox went an astonishing 82 years without winning a World Series. The "Curse of the Bambino" was the superstitious belief that said the reason for the streak was due to the Red Sox manager selling Babe Ruth to the New York Yankees between the years 1919-1920 in order to fund his broadway play (which ended up failing miserably anyway).
However, the Red Sox were able to make it to the World Series on several occasions, but failed to win. One famous example is the 1986 World Series, in which a single error caused by Bill Buckner resulted in the Sox losing the game that would have earned them the title and broken the curse.
Then, in 2004, the Red Sox made it to the World Series with the Cardinals and swept them, thus reversing the curse and ending the losing streak. They won the World Series again in 2007, 2013, and 2018.
The Red Sox continue to have a bitter rivalry with the New York Yankees.