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 gave notice that the AFL was for real, which set the stage for the leagues' later merger.
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".