(covers information from several alternate timelines)
The Pogo paradox is a causality loop in which interference to prevent an event through the use of time travel actually triggers the same event.
Such a paradox occurred in 2373, when the Borg traveled back in time to 2063 to prevent Zefram Cochrane from breaking the warp barrier. Although successful in their attempts, the alterations to the timeline led the USS Enterprise-E to intervene. In order to repair the damage done to history, the Enterprise crew was forced to assist Cochrane in making his scheduled launch, thereby ensuring the restoration of the timeline. (Star Trek: First Contact; VOY: "Relativity")