In Diplomacy you submit written orders for your units each turn on a slip of paper. Once everyone has submitted orders, all orders are revealed. The culture of Diplomacy allows various sorts of shenanigans.
What happens if you submit an order slip for another player's units?
For example, let's say you're playing Turkey and someone else is playing Germany. You submit two pieces of paper: one for Turkey and one for Germany. The German player submits orders for only Germany. When the orders are revealed, everyone can see that two different sets of orders were submitted for one country.
Back when Allan Calhamer (the inventor of Diplomacy) was playing the game, a move called a "Flying Dutchman" was sometimes used. In this move, Turkey would include among its orders an order to move a German unit. If no one noticed (or no one objected), the order would be executed as written. According to him, it was never really outlawed in the game, it just got rarer over time. What I'm asking about is only a small escalation above a Flying Dutchman, I think.