10 Star Trek Moments More Important Than You Realised

In Star Trek: First Contact, after the Borg failed their initial attempt to assimilate humanity, they launched a Sphere which travelled to the 21st century to attack Earth in the past, before the warp drive was invented.

The Enterprise followed the Sphere into the past and quickly destroyed it with a barrage of quantum torpedoes, but apparently that wasn't enough, as we learned years later in the Star Trek: Enterprise episode Regeneration that part of the Sphere survived the explosion and ended up crashing in the arctic, where it remained until it was discovered by a research team in 2153.

The surviving drones were able to escape from Earth by assimilating the research team and stealing a transport ship. The Enterprise NX-01 intercepted them and destroyed the ship, but not before they were able to send the Collective a subspace message including the location of Earth.

So although the Borg were never identified by name and only a few lives were lost, the timeline was at least slightly changed by the events of First Contact (we even saw later in the episode that Zefram Cochrane spoke offhandedly about some Humans and cybernetic aliens from the future during a speech at a university).

All of this could've been prevented if the Enterprise-E was just a bit more thorough when destroying the Sphere.