That’s a line from a Twilight book. It’s a grammar mistake pointed out by this website.
She sighed, and began whispering again.
I don’t see anything wrong with it. Is the comma the mistake?
There’s nothing wrong with that sentence.
Some would say that the comma is unnecessary so it should be removed. But it’s certainly not wrong, and it could usefully indicate a pause between the sigh and the whisper. In any case, commas are punctuation, not grammar.
People who delight in pointing out others’ grammar mistakes usually know less about grammar than they think.
There’s further discussion of the purpose of the comma in the comments below, but here’s a quote from Eats, Shoots & Leaves by Lynne Truss that explains it nicely:
“Thurber was asked by a correspondent: “Why did you have a comma in the sentence, ‘After dinner, the men went into the living-room’?” And his answer was probably one of the loveliest things ever said about punctuation. “This particular comma,” Thurber explained, “was Ross’s way of giving the men time to push back their chairs and stand up.”
Hat tip to FumbleFingers.