Small details I noticed while reading the manga or watching the anime.
Part 8 (other parts here)

The writing on Sebastian’s gravestone says: „May you be in heaven an hour before the devil knows you’re dead.“

This line comes from an Irish toast. It goes:

May your glass be ever full.
May the roof over your head be always strong.
And may you be in heaven half an hour before the devil knows you’re dead.

Apparently, in Christian beliefs the devil can only take a living human’s soul if the human willingly gives it to him. So when a good person dies it’s the last chance for the devil to claim his soul before the deceased is buried in sacred grounds, enters heaven and is forever out of reach for the devil.
The Irish toast wishes for the person to be in heaven before the devil even knows that he’s dead so that the devil doesn’t even have the chance to tempt the soul into hell.

To write such a line on Sebastian’s grave who’s a demon and who definitely won’t ever enter heaven is quite ironic.

Kitty Jay’s Grave, Dartmoor Devon. Much isn’t known about the woman buried here. According to legend Kitty Jay was an 18th century workhouse orphan who was scorned by her lover. After committing suicide she was buried with a stake through her heart. In 1860 she was buried and in years following, flowers would appear, though there were never any footprints in the snow on or around the grave. In recent years there have been several reports of a footless apparition that floats around the grave.

Jane Doe grave at Palm Memorial Park, Henderson, Clark Co, Nevada. This unidentified murder victim was found on October 15, 1980. She was murdered elsewhere and then dumped. She died from multiple head wounds. Her fingers did not match anybody on the missing person system at the time. Her identity is still unknown and the case unsolved.