Baseball: On August 12, 1951, with his Brooklyn Dodgers sitting comfortably on a 13½-game lead in the NL standings over the rival New York Giants and having just completed a three-game sweep of the latter team, manager Chuck Dressen happily if ungrammatically declared, "The Giants is dead!" By the time the season ended on September 30, the Giants had tied Brooklyn in the standings, forcing a three-game playoff series which ended in Bobby Thomson's immortal "Shot Heard 'Round the World" to give the Giants the pennant.
At a game in Philadelphia in 1989, the Pittsburgh Pirates took a 10-0 lead in the first inning, prompting Pirates radio announcer Jim Rooker to joke, "If we don't win this one, I'll walk back to Pittsburgh." The Phillies ended up rallying and won. Rooker later did a charity walk from Philadelphia to Pittsburgh.
In Game 6 of the 2003 NLCS, The Chicago Cubs had a lead with 6 outs left until they went to their first World Series since 1945. Celebrity guest Bernie Mac was called up to sing the traditional "Take Me Out to the Ballgame" during the seventh-inning stretch. Instead of singing "root, root, root for the Cubbies" like singers usually do, he sang "root, root, root, for the champions"... and sure enough, the Cubs lost the game and the following game to fall short of the World Series. (Of course, this was also the Bartman game.)
In an interview prior to the start of the 1986 ALCS regarding the pressures of postseason play, Boston Red Sox first baseman Bill Buckner said, "Your dreams are that you're gonna have a great series and win, and the nightmares are that you're gonna let the winning run score on a ground ball through your legs." Guess what happened, and to whom, a couple weeks later in Game 6 of that year's World Series?
In Britain, this sort of thing is sometimes known as the Murray Walker Curse, after the Formula One commentator who had a reputation for observing that a particular driver's victory was inevitable mere moments before their car suffered some catastrophic failure that put them out of the race. Frequently led to commentary along the lines of "Unless I'm very much mistaken... I am very much mistaken!"
Walker himself called this the "commentator's curse," saying that it happens to everybody in his profession, but pop culture particularly associates it with him.
American sportscaster Curt Gowdy was sufficiently known for this that the term "getting Curt Gowdyed" became part of the sports vernacular here in the 1960s and '70s, and is still occasionally used today.
Figure skating: Michelle Kwan, heading into the 2002 Winter Olympics having choreographed a program to the song "Fields of Gold" as the massive, overwhelming favorite. She promptly fell in the final round to slip from a gold to a bronze medal. What makes it worse? She should have known better, after having ALREADY lost as a heavy favorite in the final round four years earlier in the 1998 Olympics.
Soccer: Brazilian team Cruzeiro was in the Copa Libertadores final against Estudiantes, a team which they met in group stage. First game of the finals, a draw in Argentina. Second and final game is at home, a draw gives the title, the game during the group stage ended 3-0. So, the team president did a reunion to discuss the player's prizes, and the supporters went mad, making "Champion" ribbons — one even started paying the ticket to Dubai, where the FIFA Club World Cup would occur! In the actual game, Cruzeiro scored first, but Estudiantes went on to win 2-1.
Rugby: This advert proclaimed England to be the 2011 Grand Slam winners in the six nations. For those not in the know, the six nations is an annual tournament between England, Ireland, Scotland, Wales, France and Italy. A country wins a grand slam when they win all their matches. Trouble is, it was made before their final match against Ireland. Ireland won by 24-8.
Australian Rules Football: Speaking at the 2010 Grand Final Breakfast, following the hung parliament in that year's election, Prime Minister Julia Gillard said, "Please, we cannot have a draw!". Guess what happened?
"Commentator's Curse" (complimenting a player on their performance right before they screw up) is an established technical term among BBC commentators (also quite prevalent Stateside, esp. among commentators talking about field goal kickers). A typical exchange would be:
Guest commentator: Davis really is producing some brilliant snooker tonight.
[player misses a colour off the spot]
Main commentator: That's a nice case of commentator's curse you've got yourself there, Gerald.
In January 2012, ESPN covered a Clippers game in back-to-back weeks. In both games, color commentator Hubie Brown mentioned Clippers guard Chauncey Billups being among the top-5 leaders in free throw percentage in NBA history as Billups headed to the line. In both games, Billups missed the very first FT after Hubie put the whammy on him, something lampshaded by his play-by-play partner Mike Tirico.
In the 2008 Olympic Games, the French men's swimming team was the favorite to win the 4x100 freestyle relay and they bragged about how they would crush the US team, their main competition. Cue Jason Lezak miraculously overcoming a full-body length deficit in the last 25 meters against Alain Bernard, who had entered the race as the world record holder at 100mnote Australia's leadoff swimmer broke that record on the opening leg, to edge the French out for gold.
Before Super Bowl XLII, New York Giants player Plaxico Burress told the media his team would beat the perfect season New England Patriots by a score of 23-17. When the media told Patriots QB Tom Brady this during an interview, he laughed at the prediction. During the Super Bowl, Tom Brady, who had his best career season scoring 50 touchdowns, was mostly stopped by the Giants defense, resulting in them only scoring 14 points and losing the game that mattered the most. Since then, Tom Brady and the New England Patriots haven't lived it down. Especially, since they would lose the Super Bowl to the New York Giants again four years later.
Tom Brady: "We're only going to score 17 points?! HA HA HA! Okay!"
During his prime, heavyweight boxing champion, Mike Tyson, defended his title against a no-named journeyman fighter named Buster Douglas in Tokyo. Before the fight, it was heavily reported that Tyson didn't take Douglas seriously as an opponent and spent most of his time before the fight partying and sleeping with different call-girls every night. During the championship fight, Douglas knocked out Tyson in one of the biggest upsets in sports history.
During the 2012 NBA Eastern Conference playoffs, the Indiana Pacers were ahead of the Miami Heat 2 games to 1. With both LeBron James and Dwyane Wade playing poorly, it seemed like the Pacers would advance to the Conference Finals. However, star player Lance Stephenson decided to mock LeBron by making a choke sign. Guess what happened next? Yup, the Heat came back to win the series and the team eventually won the NBA Championship.
Moody: Marco Simoncelli leads this Grand Prix and is leading it brilliantly.[Simoncelli starts to slide] Oh NO! NO! [Simoncelli crashes] OHHHH! Simoncelli crashes,he nearly held it, and Lorenzo leads the Grand Prix!
College Football: In 2010, the University of Florida's Gators had been enjoying a string of success, earning two national championships in the past 4 years and having a 6-year winning streak against their in-state arch rivals, the Florida State Seminoles. To play up the achievement, a group of Gator fans erected this obnoxious billboard sign outside of Tallahassee, Florida (the location of FSU's campus) to taunt the Seminoles for their woes. Guess who would win the very next rivalry game between the two teams?
At the start of overtime in a 2003 playoff game between the Seattle Seahawks, and the Green Bay Packers, the Seahawks won the coin toss. Seahawks quarterback Matt Hasselbeck famously said, "We want the ball, and we're going to score." Hasselbeck proceeded to throw an interception that was returned for the game-winning touchdown.