Any Chicago Cubs fan would certainly get it; it's an act that went down in infamy in the annals of baseball. It's a picture of Steve Bartman in Chicago's Wrigley field, catching a foul ball in the sixth 2003 National League Championship Series playoff game against the eventual winners of the 2003 World Series, the Florida Marlins (their second championship in their short existence...making prepetual losers the Chicago Cubs and Boston Red Sox (who finally won last year) very upset)).
The problem is that Chicago outfielder Moises Alou had a play; he could have caught the ball of Bartman hadn't interfered from the stands. After that, the Marlins scored eight runs, the Cubs, 0. The Cubs were pretty rattled. Their 60-year losing streak continued.
Here's a better picture from the Smoking Gun Archive. The best explanation I found is on Wikipedia.