Seriously, though, this may make more sense than it looks (even though it's spending days and money that are needed filming the rest of the season). We know that Moffat likes to seed his story arc throughout the seasons. We also know that, with this season, that Moffat is blowing deadlines and behind the eightball. It may be that Moffat has only now figured out how his overarching story arc will play out across multiple episodes, so he's having to write scenes for episodes that have already filmed to make the arc work.