While doing my research on the Reign of Terror, I ran across this story on the Wikipedia page for Charlotte Corday:
After Corday’s decapitation, a man named Legros lifted her head from the basket and slapped it on the cheek. Charles-Henri Sanson, the executioner, indignantly rejected published reports that Legros was one of his assistants. Sanson stated in his diary that Legros was in fact a carpenter who had been hired to make repairs to the guillotine.Witnesses report an expression of “unequivocal indignation” on her face when her cheek was slapped. The oft-repeated anecdote has served to suggest that victims of the guillotine may in fact retain consciousness for a short while, including by Albert Camus in his Reflections on the Guillotine (“Charlotte Corday’s severed head blushed, it is said, under the executioner’s slap.”). This offense against a woman executed moments before was considered unacceptable and Legros was imprisoned for three months because of his outburst.
Footnote: Mignet, François (1824), History of the French Revolution from 1789 to 1814
I suspect that the brain would burn off the oxygen left in the bleed quicker than the man could snatch the head from the basket and give it a slap, but I could be wrong.
Wikipedia image credits for the 18th century portrait of Charlotte Corday by the artist, Jean-Jacques Hauer