A shot across the/somebody’s bows
Definition 1 of 1
Pro Tip 1/3Metaphoric Origin
Originally a naval term, signaling a warning shot without intent to harm, now used metaphorically. 
The CEO's comment about layoffs was a shot across the bow to underperforming departments.
Pro Tip 2/3Serious Consequences
Implies that if the warning is ignored, there will be serious, possibly harsh consequences. 
The ultimatum was a clear shot across the bow to the competitors.
Pro Tip 3/3Not Literal
The phrase is figurative and does not refer to actual shots or bows from a ship. 
Her public criticism was a shot across the bow to the committee.