Sometimes writers (like anyone else) just assume something is what they think it is. We do check anything doubtful, but why would we check something we know is right? Because it might not be right after all? And what if it isn't? Do we rewrite?
Sometimes, a wrong assumption can work in our favour because we can shuffle off the responsibility onto our characters. That's right... it's THEIR fault.
Barbara Allen is a case in point. It's the 1950s, and a young man calls his girlfriend Barbie Barbara Allen as an affectionate nickname. Why not? Everyone else calls her Barbie, and he'd like to be different. I'd put in the reference and decided I might check to find out the provenance of the song. I was pretty sure it was old, but what if it turned out to be a modern construct dating from 1964? I checked the date, and found it is of respectable antiquity. Then I idly looked for the lyrics.
Eek! It isn't a sentimental love-song as I'd thought. Not in the least. No. No other song would do, so I left the reference in place but later on had Barbie ask her admirer to PLEASE stop calling her that. She then explains to him as I just explained to you.
What? I didn't?
Well, Barbara Allen is the object of William's affection. He's sick with love. He's dying of love. Oops... he really is dying. Barbara Allen, appraised of this, won't go and give the lad a kind word. He dies, begging his friends to be kind to her. She hears he's dead and then wastes away and dies of remorse. So there.