I love using Joss Whedon shows as examples because what's good is very good, so what's wrong stands out.
I forgive TV shows that get historical costumes wrong because they're working fast on limited budgets. But getting historical names wrong is embarrassing, especially when a screenwriter can ask a researcher how old a name is. Case in point: Darla. When the character was introduced in the pilot, she was a vampire cheerleader without a backstory. She seemed to have been a high school student who was turned into a vampire sometime in the last three decades or so.
A good guess would be that she was born in the '60s and vampirized in the late '70s, based on Name Age Calculator: Darla: "The median living girl named Darla was born around 1963 and ranges from 45 to 58 years old." Behind the Name tells us Darla is a short form of Darlene, which is "From the English word darling combined with the popular name suffix lene. This name has been in use since the beginning of the 20th century." Wikipedia has lists of people with the same first name; all of the Darlas at Darla - Wikipedia were born in the 20th century.
But when the character became more useful to the show, the writers decided she was born in the 16th century. They did some handwaving to say it was an old Gaelic name, but that appears to be their attempt to cover up a goof.
The internet makes retconning names easier than it was when Buffy was being being made. Girl Names Starting with Dar and Girl Names that Contain "dar" have some interesting possiblities for what Darla could've been called in her pre-20th century appearances.