I read this article recently and was surprised to hear that an MMA fighter was medically cleared to fight even though they had a fractured toe. I understand Khabib is a beast but isn't there usually a medical checkup before each bout, how exactly did this slip the radar? Is it common for fighters in MMA to be cleared with injuries as significant as fractures or was this just a rare occasion?
I don't know what the UFC's exact protocols are, but to reliably find all possible fractures, you'd have to do whole-body X-rays, and I'm pretty sure they don't do that. Then you'd also have to do whole-body MRIs to find torn ligaments, etc. ...
It is important to distinguish the examinations needed to get a license from the checkup on fight day. The former may include MRI scans, ECGs and EEGs, blood tests and other procedures that would detect long-term health issues, but these tests only have to be done every couple of years, so they would not reveal any recent injuries.
From what I've seen in amateur organizations, the medical checkup on fight day consisted of visual inspection, pulse/ blood pressure, pupil dilation, and a couple of questions - "are you feeling well? Any recent concussions? Are you taking any medications?" As far as I know, in the UFC, fighters are obligated to disclose any relevant injuries (fractures, torn ligaments etc.), but I have never heard of any punishment for failure to disclose.
How often does this happen? No one knows, but UFC-level fighters are incredibly tough and pain-resistant people who fully expect to go into a fight more or less banged-up from their training camps, and the negative consequences of disclosing an injury and having an important fight cancelled may (in their opinion) outweigh the risks of keeping their mouth shut and hoping for the best.