A warp bomb was an extremely destructive form of explosive weapon, which was based on the same principles of matter-antimatter reaction that allowed warp drive. The signs of its presence could be mistaken for warp drive by sensors.
In 2259, the Eldredth faction on Kiley 279 managed to construct a warp bomb based on their planet's observations of the Battle near Xahea in 2258 with the intent to use as part of their conflict with the Palion. Federation representatives mistook the warp bomb for warp drive, and mistakenly attempted to initiate first contact procedures with the Kiley.
Until the USS Enterprise's 2259 discovery of the pre-warp Kiley's use of a warp bomb, Science Officer Spock stated that not once in the entire history of first contact had warp been first developed as anything but a drive. Spock compared it to inventing the nuclear bomb before particle physics; that it could happen but it simply never had.
Realizing the apocalyptic threat the warp bomb posed, Captain Christopher Pike chose, in violation of General Order 1, to steer the Kiley in a less destructive direction. (SNW: "Strange New Worlds")