Earth Shattering Kaboom: The Ulijen Disaster.  If you’re going to build an experimental power plant that taps power from your sun using a wormhole, don’t build it on the planetary surface!

Also, relativistic kill vehicles and really big strangelet bombs, or even antimatter bombs; and in theory, some other esoterica like twist-pinch bombs and causal weapons.  All of which are utterly, utterly illegal to use (which doesn’t stop everyone with a sense of paranoia from maintaining a relativistic deterrent fleet, of course) under pain of serious obliteration.