It's something of a sports cliche to say that you should never give up because a comeback is always possible. In 1987, during a fourth round Wimbledon match, Jimmy Connors was getting thrashed by Mikael Pernfors, 1-6, 1-6, 1-4. All Pernfors had to do was to hold serve twice and he would have had an easy straight sets victory. Connors came back and won in five sets.
In 1993, the Houston Oilers (now known as the Tennessee Titans) were absolutely annihilating the Buffalo Bills, 35-3, moments in the third quarter. Worse yet for Buffalo, its starting quarterback Jim Kelly was out with an injury. Backup quarterback Frank Reich led the Bills to the biggest ever comeback in NFL history, winning 38-35.
In 2004, the Boston Red Sox lost the first three games of the American League Championship Series to the hated rival New York Yankees. No MLB team had ever won a best of seven post-season series after losing the first three games. On top of that, Boston's bullpen was exhausted from the previous three games. Yet, Boston won four games in a row (en route to the World Series championship), playing two extra-inning games along the way.
Still, there was an ample amount of time available to overcome the disastrous starts in those examples. This race is six years old but I just discovered it today; it's a 600 meter race, so we're really talking about a minute or so to make up the time lost due to a costly mistake. Check it out: