The UFC has had fights put on regularly for nearly thirty years now. However, not everything goes flawlessly. While the biggest fights in MMA history happened in the UFC, the promotion also saw a lot of fights slip through their fingers. Most of these were due to injuries that were sustained during the athlete's training camp.

RELATED: 10 MMA Fights That Fans Thought Would Be Close (But Were One-Sided Beatdowns)

That being said, that's not the only time that a fight will get canceled. Actually, the sport of MMA has had many bouts get canceled as early as the day of the event, with some even being removed off the card just minutes prior to the fight occurring.

10 Max Holloway vs. Khabib Nurmagomedov (UFC 223)

One of the biggest what-ifs in MMA history is the bout between Max Holloway and Khabib Nurmagomedov at UFC 223. 'Blessed' took the fight on six days' notice in an attempt to become a two-weight UFC champion after Tony Ferguson was forced out of his matchup with 'The Eagle'.

While Holloway came in on such short notice, many assumed he'd be fine making weight because he's a career featherweight. That being said, he was forced out of the card on weigh-in day, due to a bad weight cut that had Holloway so drained he couldn't make the scale.

9 Stefan Struve vs. Matt Mitrione (UFC 175)

Stefan Struve was temporarily forced into retirement in late 2013 due to an enlarged heart. Months later, the heavyweight was actually cleared to return and was matched up with fellow contender Matt Mitrione at UFC 175 in July 2014.

During the first bout of the main card, Struve reportedly had a cardiac episode and passed out backstage. Luckily, the big man ended up being okay and just had an elevated heart rate, but his fight was canceled as a result of his fainting.

8 Matt Mitrione vs. Fedor Emelianenko (Bellator 172)

Matt Mitrione just can't catch a break seemingly. Just two-and-a-half years after his fight with Struve was canceled during fight day, his bout with the legendary Fedor Emelianenko was called off in a similar fashion.

RELATED: Ryan Bader Vs. Fedor Emelianenko (& 9 More Of The Weirdest Fights Of Fedor's Career

Right as the prelims were getting underway, Mitrione was forced to the hospital with kidney stones. Due to the incident, he was forced off the card and the fight was postponed until June at Bellator NYC. Luckily for 'Meathead', he made the most of his second chance and won via knockout.

7 Trevin Giles vs. Kevin Holland (UFC Vegas 5)

&copy; Tom Szczerbowski-USA TODAY Sports<\/p>\n"">

UFC Vegas 5 was the shortest UFC card in six years due to many fight cancelations. One of those cancellations came just moments before the bout was supposed to happen.

Trevin Giles was set to fight Kevin Holland on the card, and while the former was about to make his entrance, he fainted due to heart irregularities. Following the fainting incident, the fight was called off.

6 Renan Barao vs. T.J. Dillashaw 2 (UFC 177)

Renan Barao and T.J. Dillashaw ended up fighting for a second time, but it didn't come at UFC 177. The fight was originally set for the main event, but Barao was replaced with Joe Soto on one day's notice.

The Brazilian was forced out of the bout due to him fainting while weight cutting. As he fainted, he fell back and hit his head on a bathtub, giving him a concussion. Due to the injury, he was forced out just one day before the bout.

5 Ken Shamrock vs. Kimbo Slice (EliteXC: Heat)

A pullout that ended up killing an entire fight promotion came in October 2008. Former UFC champion Ken Shamrock was set to face Kimbo Slice in the main event of EliteXC: Heat, in a huge matchup for the rising promotion.

During the pre-fight warmups, Shamrock accidentally got headbutted by a coach. The strike opened up a huge cut and was forced out of the fight. The lightly-regarded Seth Petruzelli was moved from the prelims to the main event and knocked out Slice in seconds.

4 Tony Ferguson vs. Khabib Nurmagomedov (UFC 209)

Yes, these two had many canceled fights. However, none came closer to happening than their UFC 209 co-main event bout. The two men were set to clash for the UFC interim lightweight title, and up until weigh-in day, everything was going according to plan.

RELATED: 10 Biggest What If Questions In UFC History

That's when everything went wrong. While Ferguson made weight successfully, Nurmagomedov had a weight cut gone wrong. He had medical complications as a result of the botched cut and was forced to stay in the hospital, forcing the bout to be canceled.

3 Robert Whittaker vs. Kelvin Gastelum (UFC 234)

Robert Whittaker vs. Kelvin Gastelum was set for the main event of UFC 234 in his home country of Australia. However, hours before the fight, the UFC middleweight champion went to the hospital due to stomach pain and pulled out.

The move ended up saving his life. Whittaker had a hernia and collapsed bowel and was forced to undergo immediate surgery. Had he proceeded with the contest, it's possible that it would have been fatal.

2 Amanda Nunes vs. Valentina Shevchenko 2 (UFC 213)

\n&nbsp;"">

The main event of UFC 213 was set to be a rematch between Amanda Nunes and Valentina Shevchenko. The fighters made weight, and everything seemed good to go, up until a few hours before the doors to the T-Mobile Arena opened.

The UFC bantamweight champion seemingly came down with an illness and was hospitalized. Thankfully, the two fought a few months later, with Nunes winning via split decision.

1 Harold Howard vs. Royce Gracie (UFC 3)

UFC 3's one-night tournament was seemingly set up for a Ken Shamrock and Royce Gracie rematch. Instead of that, fans got to see Harold Howard vs. Steve Jennum, as both Shamrock and Gracie had to pull out at different points of the tournament.

However, it was Gracie's cancelation that gave Harold Howard an automatic bye into the finals. Following his war with Kimo Leopoldo earlier in the night, the Brazilian pulled out of his semi-finals bout due to fatigue and injury.