warning shot
1. Literally, the firing of a weapon to signal an impending attack and/or to urge the opponent into surrender. We're almost at the shore—send up a warning shot before we land.
2. By extension, something used to signal future events and/or to urge people into a particular course of action. This petition is just a warning shot, saying that we'll be arrested if we keep playing our music so loud. I say, turn it up!