The pocket watch
So Old Elise gives Richard the pocketwatch. He goes back in time and gives it back to Young Elise, who then keeps it for 50 odd years before giving it back to him once again. This same watch has been passed back and forth between them. Two obvious questions arise.
1. Watches are not indestructible. Eventually that watch will be centuries old. Wear and tear will break that thing down and it will fall into disrepair. What happens then when the watch can no longer be exchanged?
2. Where did that watch originate from in the first place? Who had it first? What chain of events kick started this repeating cycle?