Instant death, right? Wrong. This phenomenon actually occurs more frequently than you’d expect after cases of trauma, with many people surviving. In fact after a car accident, a woman spent 1 year diagnosed with whiplash before someone finally did an X-ray and realised what the crap was going on. It went on for so long that the bones in her spine began to deteriorate and her neck collapsed to the point of her chin resting onto her clavicle - this then lead to sores and infection. This however is a rather unfortunate scenario; in most instances it’s caught right after the accident and reattached immediately, with both kids and adults having survived it. But still, eeeesh!
So how does it happen?
This nasty scenario occurs when the atlanto-occipital joint is dislocated (this a synovial joint which attaches the skull to the spine - you can see a clear detachment of these in the image). Most documented cases of it occurred during road traffic accidents, either as a driver or pedestrian. There have also been some reported cases during childbirth. All of these survivors have of course been very lucky to not have suffered any accompanying spinal cord/nerve damage because that it a pretty sure ticket outta here.
Another example of internal decapitation worth mentioning is hanging. I’d hope to not survive in that instance because you end up asphyxiating instead. Nasty!