The curse that prevents the Chicago Cubs from ever winning the National League Pennant. Contrary to what a previous user wrote, the curse began in 1945 when a tavern owner tried to bring his pet goat into Wrigley Field during Game 1 of the World Series because it was a good luck charm. Security didn't allow it and as they escorted him out he declared that the Cubs would never again win the pennant. And they haven't.
"Did you hear Sammy Sosa hurt his back while sneezing?"
"Yeah, it must be that billy goat curse again."