Incoming! You never hear the one that gets you.
Great idea for an SF tale!
Interstellar artillery duels with massive lasers. Of course, targeting would be a problem--it might take years or even centuries for your fire to hit it's tiny, moving target. Still, there would be no possible warning. Even if you had trip-wire sensors in the line-of-sight, they would not have time to alert home base a bolt was on its way (max speed = c).