If it's by Mozart it means "Watch out!" You never know with him. More often than not it is a deceptive tactic inserted into the line to tease you into thinking you know what you're doing when it turns out that it was put there merely to call attention to a line by interrupting it.
He has been known to do this sometimes at double bars; also between movements.
I know: you were taught, as I was, that phrases had beginnings and ends and that rests were one indication of punctuation. Ah me! another lesson to outlive.
