So many things can go wrong. That's the reality for any given horse in any given race.
Maybe he's burned out from training too hard. Maybe he has a sore hoof. Maybe it's raining and he doesn't like the mud. Maybe he stumbles coming out of the gate or another horse bangs into him. Maybe his jockey doesn't devise the right tactical plan. Maybe he's simply tired compared with the rest of the field.
Multiply those possibilities over three races packed with the best 3-year-old thoroughbreds in the world and it's easy to understand why we haven't had a Triple Crown winner in 30 years. It was never meant to be as easy as it seemed in the 1970s, when three horses did it in six tries.
Horses that pull it off must be fast, but they also must be versatile enough to deal with all of the eventualities. And often they have to be a little lucky.
Big Brown will try to become the 12th Triple Crown winner Saturday at Belmont Park. Six other horses had that chance between 1997 and 2004. They failed for a variety of reasons.
"There are multiple reasons why horses haven't won the Triple Crown," Hall of Fame jockey Jerry Bailey said. "Sometimes it was physical things, sometimes the distance, sometimes pilot error."
The first factor always mentioned at the Belmont Stakes is the 1 1/2 -mile distance. If the Derby is feared because of crowded fields and the Preakness because of its tight turns and the short turnaround after Churchill Downs, the Belmont is known for being the longest race of the series. Many favorites have surged to the lead only to lose steam and fall to upstarts.
Often when that happens, observers blame the jockeys.
Hall of Fame jockey Gary Stevens still blames himself for charging too early on Silver Charm in 1997.
Kent Desormeaux, who will ride Big Brown, faced similar doubts after Real Quiet tired and lost by a nose in 1998.
Trainer D. Wayne Lukas said rider Chris Antley made a mistake by allowing Charismatic to duel for the lead early in the 1999 race. He finished a distant third.
Five years later, Stewart Elliott received similar criticism after Smarty Jones responded to early speed challenges from Eddington and Rock Hard Ten and tired before the finish.
A late fade can be set up by a poor start.
Trainers of favored horses often say a clean break from the gate is the key to victory. This truism caught up with War Emblem at the 2002 Belmont. He had always been a nervous starter, but jockey Victor Espinoza thought everything was fine when the colt entered the gate smoothly.
Just after he broke, however, he lurched so violently that he was inches from throwing Espinoza and hitting the track. Only a collision with Magic Weisner jolted Espinoza back into the saddle. Despite that, War Emblem, a speed horse, had to fight too hard for the lead and tired down the stretch.
Asked whether the bad start cost him the race, Espinoza said: "Definitely. The horse was so ready for that race. It was just one of those things. If he stumbled just a little, I think he'd have a little [better] chance."
Then there are factors completely outside the control of horse or jockey.
Funny Cide seemed perfectly set up to win in 2003, coming off a dominant performance in the Preakness and returning to his home track. But Belmont was not the track he was used to. A day of heavy rain had turned supple dirt into thick mud. Funny Cide did not like mud.
He took the lead early but simply never looked comfortable on the sloppy track, trainer Barclay Tagg said.
"I thought he ran as hard as he could," Tagg said. "But as the rain kept coming all day, I knew we were in trouble."
Funny Cide also started at a disadvantage because his chief rival, Empire Maker, had skipped the Derby and started the Belmont as a fresher horse. This has become a common obstacle for would-be Triple Crown winners.
Elite horses that fail to win the Derby often skip the Preakness, only to return for the third leg of the series. Some talented horses simply swoop in for the Belmont after skipping both of the first two legs. Big Brown's biggest threat, Casino Drive, did so.
Conversely, a horse attempting to win the Triple Crown is usually completing the most grueling stretch of his career. That's certainly the case for Big Brown, who had run only three times entering the Derby.
Fatigue and fresh competition are the reasons trainer Rick Dutrow has said the Belmont will be Big Brown's greatest challenge. There's also the matter of his cracked hoof, which apparently won't keep him from racing but could be an X factor, according to some experts.
Spectacular Bid was considered just as untouchable a favorite in 1979. Until he stepped on a safety pin the morning of the race.