Basically, the characters come across an NPC who knows them, but whom they do not know in any way. In my original incarnation, they meet the character as he dies--glad to be surrounded by friends and saying something like, "So this is what you were trying to tell me," but then see him a few days later hale and healthy. The idea is that they would continue to meet him in different parts of their journey, and I could use him to give them hints about what was coming up in the adventure.
As I mentioned, the adventure was short-lived, so they only got to meet the character twice, but I was already having trouble pulling off the concept as I was getting myself confused about what who would know when. Still, it was an amusing idea. If I did it again, I wouldn't start with the character's death as that cast a darker pall over their second meeting than I fully intended. That, and the concept has now already been done with the aforementioned River Song.
If I did do it again, I would like the character to be puzzled by their first meeting--expecting them to already know the character. I have a couple of different ways that this could work. In one, the character just lost track of events: "You don't know me? Wait! What's today's date? No! I had something special all planned! I can't believe I screwed this up! Here, sorry, it's nice to meet you. We're going to the best of friends. Trust me, I know!" In another, something's gone wrong with time: "No! This isn't our first meeting! You should already know who I am by now. Don't you remember last week's brunch? What? Oh, no! That brunch was amazing; there's no way you'd have forgotten it! Something's gone terribly wrong!"
I don't think that I really want to play with time travel in this upcoming game as it adds an extra level of complication to everything, but it's a cute idea.