The fridge.. or the TARDIS....
Fair one.
At least Moffat's admitted they saddled Jenna Coleman with a storyline that was unfair to her as an actress, and that the character as-written wasn't strong enough to carry. Which really makes me ask 'Why did you bloody do it, then?'
I mean, not every companion has to have some weird stuff going on, so why not drop the 'impossible girl' BS and let her just be Clara?
Yes, she'd appeared in a previous ep and that created a bit of a problem, but it wouldn't really have been that hard to work around to get back to that point. It might have meant that she couldn't originally be from our time, but so what? Past companions haven't even been from Earth, let alone the present time (Susan, Leela, Nyssa, Adric (might have been from Earth, but not the then-present), and iirc Turlough - and there are probably more).
Even if Clara had been from present-day London, however, there's nothing that says she has to return to her own time. She could have got stuck in humanity's far future, or opted to leave the Doctor to live in a future that offered more to her than a return to her own time could (perhaps she could have been 'impossible' in that sense; she'd have been out of her own time, affecting a future that she shouldn't have been a part of - but at the same time, always was and would always have to be, at least to the Doc who has knowledge of what's supposed to happen, where and when, etc). From there, she could have joined some kind of mission, which then crashed and was discovered/was brought down by the Daleks, and away we go. The circle is complete.
But with all that said, Moffat does like a complicated, 'clever' storyline. He's obviously forgotten, or never learnt, that the cleverest stories are actually pretty simple. It's not about twists and turns and micro-hints - or it isn't just about those things (and less is more) - it's about the story and how well you tell it.
(Case in point: the mummy ep was simple, and all the more effective for it. Though Blaine the Mono was perhaps unnecessary and Clara could/should have been sent on her way after her 'Push off' speech at the end of the previous ep.)