Probably something like "It's all inside your head". I also love to imagine the message having good intentions, but the world falls into anarchy because people will start theorising it's the end of the world.
Probably something like, "Run." Especially to the people that watch too many horror movies (like me >:)) or something like the hot, cold game to where I get people going in circles every single day.
If it didn't have to be one sentence, I would do a conversation between two people discussing whether or not to shut down the simulation, then panicking about how the broadcast it to the whole simulation.
