NOTES:
Now for that promised weird stuff that didn't quite fit in to the other categories.
Let's take a simple premise - that it is possible for a technological species to create vehicles with relativistic velocities. Within a few decades, we might be able to create such vehicles. They have the following characteristics:
Quick travel times - with a few decades of travel one could reach dozens or hundreds of other star systems.
High destructive potential - if one hit a planet, it would be worse than a huge asteroid strike, just because of the speed of the impact.
No conceivable defense - Defense against such a vehicle is very difficult because the huge momentum such an object has would be impossible to deflect, and mere detection would be difficult because the object is right behind the its own light.
Killing Star (by Pellegrino and Zebrowski) suggests that the possibility of such vehicles might lead all intelligent life forms to try to destroy all other intelligent life as soon as it's detected due to simple prudence. This is one of the key strategies of SF - taking a simple premise and finding the most outrageous (and horrifying) but plausible consequences.