Khabib
When the UFC signed him in late 2011, he had 11 fights at welterweight and had only weighed in twice at lightweight, one of which was as a teenager.
His father convinced the UFC brass that Khabib could reliably make lightweight. This proved to not be true. Khabib officially missed weight in the UFC, had several fights at catchweight, pulled out of an interim title fight at UFC 209 less than 4 hours before weigh-ins because he was going to miss weight again, fought only 8 times over the last 7 years of his career, and struggled mightily to get the scale to say 155 in his last two fights.
It is a shame, because had Khabib stayed at welterweight, he would have had the opportunity to build a truly amazing resume.
When Khabib signed to the promotion, GSP was 16-2 in the UFC, Jon Fitch was 13-1-1 in the UFC, and Nick Díaz was on an 11-fight winning streak.
In the first few years of Khabib's UFC career, Johny Hendricks emerged at a top welterweight (every media member thought he beat GSP, then he won the title and defended it after GSP retired instead of the rematch), Robbie Lawler became champ and defended the belt a few times, as did Tyron Woodley.
During this time, Carlos Condit, Rory MacDonald, Demain Maia, and Stephen Thompson proved to be top contenders.
More recently, Kamaru Usman and Colby Covington have established themselves as among the all-time greats at welterweight. Undefeated prospects Khamzat Chimaev and Yaroslav Amosov (currently outside of the UFC) could potentially join them.
By continuing to fight at welterweight as he had been doing, instead of changing course and trying to make lightweight, Khabib could have fought more often than just 8 fights in 7 years.
Had Khabib won in impressive fashion at welterweight in the UFC and kept winning, he would have had the opportunity to fight all of those top welterweights in their primes.
Saint-Pierre
Fitch
Díaz
Hendricks
Lawler
MacDonald
Condit
Woodley
Maia
Thompson
Usman
Covington
Chimaev
Amosov (if the UFC signs him when he becomes a free agent)
Instead, by trying to make lightweight, which resulted in only being able to fight sporadically, Khabib has a much thinner resume.
Gaethje
Poirier
McGregor
Dos Anos