TIL that experiments have shown that the brainwaves corresponding to consciousness in mammals can survive decapitation for several seconds. This hasn't been directly tested in humans, but there is no clear reason why this wouldn't also apply to us, such as during decapitation by guillotine. User ARTexplains, in the Today I Learned (TIL) subreddit, 14 Feb 2018
PLOS ONE: Decapitation in Rats: Latency to Unconsciousness and the ‘Wave of Death’ User Yangel, in the Conjuring the Apocalypse subreddit, 21 Nov 2015
TIL in 2011, Dutch scientists hooked an EEG machine to the brains of mice fated to decapitation. The results showed continued electrical activity in the severed brains for nearly four seconds, indicating conscious activity. Studies in other small mammals suggest even longer periods. User blihk, in the Today I Learned (TIL) subreddit, 20 Jul 2015
"The question whether decapitation is a humane method of euthanasia in awake animals is being debated." User weinerjuicer, in the Biology subreddit, 28 Jan 2011