Joe Namath's famed guarantee of a Super Bowl victory by the 1967-1968 New York Jets was seen as this, as the AFL to which the Jets belonged was perceived as an inferior league to the NFL. By following through with that guarantee, the Jets created the Super Bowl (which to that point had been something more akin to an All-Star game, with the NFL Championship considered to be the true Championship game), and helped dispel the notion that that the AFL was an inferior league and made the impending merger more palatable to long-time football fans.
Boston Red Sox DH David Ortiz is quite good at these, but none may be more memorable than the one he gave at the first game back at Fenway Park after the Boston Marathon bombings.
"This is ourfuckingcity. And nobody's gonna dictate our freedom. Stay strong."
In a time where NBA teams hadn't won back-to-back titles in 19 years, coaching legend Pat Riley of the Los Angeles Lakers said to the fans "Next year, we're gonna win it again!" They did.
Despite being a top seed in the 2011 NBA Playoffs, the seven other teams in their conference openly expressed a preference on going up against the Dallas Mavericks (who would go on to win the title), believing they were an easy mark. Mavericks guard Jason Terry acknowledged the conceit and welcomed it:
When the Miami Heat faced a 0-3 series deficit against the Michael Jordan-led Chicago Bulls in the 1997 NBA Eastern Conference Finals, Heat player Alonzo Mourning guaranteed a Game 4 victory and that the series would not be a full sweep for the Bulls. He delivered on his promise and the Heat would go on to win that game, even though they ultimately lost the series.
Speaking of the Heat, after LeBron James joined the team in the summer of 2010 after his (in)famous "Decision" to leave the Cleveland Cavaliers, he bragged that he planned to win eight NBA championships with the Heat at a welcoming party before even playing a single game. It remains to be seen whether he will deliver on this boast, although as of 2013, he is a quarter of the way there.
"Marvelous" Marvin Hagler, before his match with Sugar Ray Leonard, commented on Leonard's troubles with a detached retina, saying "If he's fool enough to fight me, I'm fool enough to rip his eye out".
1994 Stanley Cup playoffs, Eastern Conference Final. The New York Rangers, 54 years removed from their last Stanley Cup victory (the longest Cup drought ever), just dropped Game 5 of their best-of-seven series against the New Jersey Devils 4-1 on home ice. Rangers captain Mark Messeir then came out and publically declared the Rangers would win Game 6 at Brendan Byrne Arena (the Devils' home arena) to send the series back to Madison Square Garden for a deciding Game 7. After two periods the Devils were up 2-1, then Messier himself scored a natural hat trick (three goals in a row) in the third period and the Rangers win Game 6. Rangers win Game 7 in double overtime, then edge the Vancouver Canucks in seven games to win the Cup and end the Curse of 1940.