I should write up the convention game I ran a couple of years ago (at the Discworld Convention 2012, as it happened) using the 5e playtest. That was a longish game (somewhere around six to eight hours, I think) and I knew that some of the players would not be able to be there for the whole thing. It was also run in the bar, so sometimes people would go to order food and drink.
I had a small scenario where the party met in the traditional cliche inn late at night, somewhere on the more isolated reaches of a major trade route. Add some oddities: the top floor of the inn has been hired by a group of travellers, but all the stairs to it seem to have vanished.
Whenever anyone leaves the group, turn to everyone still there and say: "The person that left was never here. You have no recollection of them whatsoever. Anything they did in the past stays done - you presume that someone else must have done it. Possibly yourselves."
If anyone comes back or joins the group part-way through: "That person has been present all the time. You have clear memories of them being here". For those who left the game previously (and thus ceased to exist) all memories of them being present are fully restored.
The players are told that their character don't notice anything strange - until things start to contradict in play. It doesn't take long (and a couple of fights with some cultists and their guards) for causality to be frayed quite a bit just by people going to buy drinks, let alone when someone vanishes for an hour and comes back. The tipping point for the group I ran through this was when the entire party present had distinct and contradictory memories of two different people tying up a prisoner, unaided. Once the characters compare notes it's clear that something strange is happening, once they were aware, I started having them notice that the inn was changing randomly whenever they weren't looking at it. Anything not being observed was likely to change slightly by this point.
Eventually they find a way up to the top floor and beat some cultists and other nasties before they can complete a rather unpleasant ritual.
It's a really fun scenario and, as far as I can see, it's only possible to run it in an environment like a convention bar. Which is a shame, because I don't get to run games in those very often
