A top boxer might fight three or four times in a year at the very most. Twice is more common. So a single disappointing performance can sink a fighter's stock for an entire year or longer.
Not all of the fighters on this list even lost, and of those who did, most will come back strong. At least a few of these fighters are going to be stars for years to come.
But all of them would have liked to see things go a little bit differently in 2013.
This was the heavyweight title fight that seemed like it was never going to happen, even though Alexander Povetkin has been the top heavyweight in the world not named Klitschko for most of the past five years.
But this year, Russian promoter Vladimir Hryunov put up a record $23 million purse bid, and Klitschko-Povetkin finally happened in Moscow in October.
Klitschko won in a blowout on the scorecards, but the fight was a major letdown. Outside of Round 7, when Klitschko dropped Povetkin twice, the champion spent most of the fight jabbing and clinching.
The HBO team was already panning the fight, even as they broadcast it. It might be a while before the heavyweight champion fights on American television again.
Delvin Rodriguez came into his fight with Miguel Cotto last October as a career journeyman/fringe contender. Virtually nobody picked him to beat the three-division world champion.
But fans who have followed the sport the past several years know Rodriguez is a smart professional who always shows up well prepared. He's taken part in some terrific fights and has been on the raw end of some lousy decisions.
The fight with Cotto represented Rodriguez finally getting a high-profile opportunity. There was some sentimental desire to see him take advantage of his big break.
Even though most expected Cotto to win, a lot of fans felt Rodriguez would be able to make it a competitive fight. Instead, he simply provided Cotto with a stepping stone back into the spotlight.
When Chad Dawson defended his light heavyweight title against Adonis Stevenson last June, it was his first fight back since losing by a brutal Round 10 TKO to Andre Ward in September 2012. But Dawson was still the lineal champion at 175 pounds and was widely viewed as the top dog there.
Stevenson was supposed to be the fight that got Dawson back on track. Instead, Stevenson never even let the champion get started.
Once again showcasing his otherworldly power, "Superman" Stevenson knocked Dawson out cold in the opening round.
Lucas Matthysse entered this year as one of the sport's hottest fighters, and a pretty good percentage of fans and writers picked him to beat Lamont Peterson last May. But Peterson was the reigning IBF light welterweight champion, fresh off of an impressive TKO of Kendall Holt in February.
Peterson had lost just once in his career, to Timothy Bradley, and there were certainly knowledgeable boxing fans who expected him to prevail over Matthysse.
What actually happened came as a shock to nearly everybody. Matthysse jumped all over Peterson, who has been a notoriously slow starter throughout his career.
Peterson was down once in the second and twice in the third before referee Steve Smoger stopped the action.
Peterson is still the IBF 140-pound champion, and he'll defend his belt against undefeated Dierry Jean in January. But it will take a pretty big performance for people to view him as truly relevant again at the top of the weight class.
When Sergio Martinez fought Martin Murray last April, it was the world middleweight champion's triumphant return to his native city of Buenos Aires, Argentina. But while he did manage to escape with the victory, it was hardly the performance his fans and promoters would have preferred to see.
Fighting outside in a rainstorm, Martinez struggled to maintain his normally feline balance. He was dropped by Murray in Round 8.
Referee Massimo Barrovecchio's failure to rule properly on a second knockdown for Murray was ultimately responsible for Martinez's winning scores. After the fight, Martinez had to go in for yet another surgery, putting him out of commission for the rest of the year.
Martinez will return in 2014, and I wouldn't be surprised if he has at least one or two more great fights left. But he'll be 39 and coming off of multiple surgeries in recent years, so nothing is guaranteed at this point.
Seeing an aging champion who has been as great as Martinez struggle the way he did to win against Murray was one of the year's biggest disappointments.
In June, Adrien Broner beat Paulie Malignaggi by split decision to capture the WBA welterweight title. The victory made Broner a three-division world champion at just 23.
But it wasn't the performance Golden Boy had hoped for from the young star they've already tapped to be Floyd Mayweather's heir. While I thought Broner did indeed do enough to win the fight, it was extremely competitive.
For Broner's legion of haters, it was close enough for them to feel justified in crying, "Paulie was robbed!"
Still, Broner escaped with the belt and the win. If he can manage an exciting victory against Marcos Maidana this Saturday, a lot of the disappointment from this summer will be long forgotten.
In the end, it didn't seem so surprising after all when Floyd Mayweather beat Saul Alvarez in a completely one-sided fight last September. But it was still a disappointment.
This was a fight that set pay-per-view records. Against their better judgement, a lot of fans had convinced themselves that maybe the redheaded kid from Mexico could do the unlikely and force Mayweather into a competitive fight.
Many of Canelo's diehard fans truly believed he would win.
Instead, Alvarez was trounced as thoroughly as anybody Mayweather has fought recently. C.J. Ross forever disgraced herself by scoring the fight a draw, but all three judges gave Alvarez too many rounds.
Nonito Donaire entered his highly anticipated showdown with Guillermo Rigondeaux ranked between No. 3 and No. 5 on nearly every pound-for-pound list. In 2012, he had been the Fighter of the Year.
The four-division world champion was a major star for HBO and Top Rank. He was a fighter they might try to build a pay-per-view around, if the proper opponent could be found.
Rigondeaux was held in high esteem by the boxing purists of the world, and some fans did pick him to win. But few expected him to handle Donaire with such ease.
Donaire has been one of the sport's most exciting offensive fighters in recent years. To see him totally neutered by Rigondeaux was a big disappointment to many observers.
In May, Abner Mares stopped Daniel Ponce De Leon by Round 10 TKO to become a three-division world champion. Mares' list of opponents in recent years had been as good as anybody's in the sport, and he was an obvious pound-for-pound top 10.
In the week before Mares faced Jhonny Gonzalez last August, Golden Boy CEO Richard Schaefer even gave interviews stating that Mares deserved to rank No. 2 behind only Floyd Mayweather.
So it was a major disappointment for many people when Mares got starched in the very first round by the veteran Gonzalez.
Mares is an exciting fighter with a growing fanbase and should not be counted out yet. He's got a rematch set with Gonzalez for February.
Julio Cesar Chavez Jr. returned to action against Brian Vera last September after serving a suspension for testing positive for THC following his loss to Sergio Martinez in September 2012.
The fight had already been delayed due to an alleged cut suffered by Chavez in training. Vera made no secret of his belief that Chavez's camp was stalling due to Jr.'s inability to get his weight down.
And the weight for the fight was, in fact, renegotiated numerous times. At the last minute, Chavez's camp had to negotiate for a further increase from the super middleweight limit of 168 to 173.
Vera had been selected as an opponent because he is an aggressive pressure fighter with limited knockout power. He was expected to make Chavez Jr. look good. There was already talk going around about a potential superfight between Chavez and Andre Ward.
But once the fight started, Vera completely outhustled Chavez, who appeared fatigued early in the fight and seemed to have no strategy aside from loading up for the big punch.
When the fight appeared to be very close in the late rounds, Chavez Jr. devoted the lion's share of his effort toward complaining to the referee. The legend's son escaped with a unanimous decision, but it will be on most writers' short list for the worst decision of the year.