Originally Posted by Bad Robot
All the accounts are quite anecdotal -and gruesome- but here's a few:
(to be taken with a pinch of salt)
In 1905, [Dr Gabriel Beaurieux] arranged to attend the execution of the murderer, Henri Languille. Shortly after the blade severed Languille’s head, Beaurieux noted a frightening observation:
[T]he eyelids and lips of the guillotined man worked in irregularly rhythmic contractions for about five or six seconds. [After several seconds], the spasmodic movements ceased…It was then that I called in a strong, sharp voice: “Languille!” I saw the eyelids slowly lift up, without any spasmodic contractions – I insist advisedly on this peculiarity – but with an even movement, quite distinct and normal, such as happens in everyday life, with people awakened or torn from their thoughts. 
Fascinated, Beaurieux called out the victim’s name again, and again, Languille’s ‘eyelids lifted and undeniably living eyes fixed themselves on mine with perhaps even more penetration than the first time’. 
On the third attempt, there was no response.
Some experiments with severed heads | A Blast From The Past
This one mentions Lavoisier as pointed out by someone else.