https://www.quora.com/When-a-persons-head-is-cut-off-with-a-guillotine-sword-or-axe-does-the-blade-cut-the-vertebrae-bones-or-does-it-go-between-them-Can-the-vertebral-bones-ruin-an-execution
Beheadings can certainly go awry, but this is often due to a dull blade, oblique blow, or thick neck muscles, as occurred to Louis XVI in 1792.