Many obvious classical allusions in the Harry Potter novels stem from JK Rowling's undergraduate degree in Classics at the University of Exeter, causing classical intertexts to rub shoulders with the likes of The Worst Witch and the Narnia books. However, one of the most distinctive elements of the stories may not be original either.
'According to report, the Mithridates who made war against the Romans...was by far the greatest drinker among his contemporaries, and so was nicknamed Dionysus [which is unlikely to be the real reason]... Actually, when he was a baby, a bolt of lightning burned his swaddling-clothes, but did not touch his body, except for a trace of the fire which remained upon his forehead as a youth and was concealed by his hair.'
(Plutarch, Symposiaca 1.6.2, Loeb translation).
Plutarch goes on to say that as an adult, King Mithridates survived another lightning-bolt unharmed (except that the arrows in the quiver hanging over his head were charred). The people called him Dionysus, hinting that, like the god who survived incineration by lightning as a baby, Mithridates had been shown to be divine.
This explains why the scar on Harry Potter's head, caused by the survival of Voldemort's 'killing curse' when he was a baby, is in the shape of a bolt of lightning. Fans have not previously been able to explain this.