I was surfing around some Doctor Who sites after coming up with my mad idea, to see if anyone else might be thinking the same thing or if there was any evidence to support it, and I found this picture, linked again at the bottom:

http://excoboard.com/forums/24571/user/216967/324532.jpg

In that picture, there is someone else at the lake scene at the beginning of the series. Someone is hiding behind a nearby shed, watching the events that unfold. I went to look at the episode, and you can clearly see someone back there when you know to look. It's clearly a person, as a hand reaches out to one side and the figure shifts slightly more into view.

That could be a production error, but this is a Moffat episode. It's the same sort of thing he did with the Jacket Doctor in season five, and that wasn't a production error. Something in the background, nothing calling it out, and your focus is intentionally being held elsewhere.

I'm forced to conclude that someone else really was there, watching. Who was it, though? I have non idea. It could be another version of the Doctor, or it could be anyone else at all in the entire universe.

I'm also wondering now if perhaps this latest episode wasn't a setup for what might happen later. Perhaps the Doctor can avoid his own death (which he knows is coming) by creating a paradox that erases his future self (the one who dies), allowing his younger self (our current Doctor) to survive into a brand new future.