We deliberately don't enable target="_blank" for hyperlinks within Prismic. But if a content editor copy/pastes a pre-built link with the target attribute filled in (from e.g. a Word doc), this will make its way through to the JSON response.
This obviously isn't too big of a deal because when we notice it, we know to re-add that link manually. But it can go unnoticed and would be nicer if the attribute never made it through in the first place.