Star Trek: Enterprise and the Kelvin Timeline
It's canonically established that Star Trek: Enterprise is set within the Prime Universe, but there's an argument to be made that it technically belongs in both.
The Kelvin Timeline was introduced in the 2009 Star Trek reboot movie, directed by J.J. Abrams and starring Chris Pine as James T. Kirk. This movie replaced the original cast with new actors, but instead of a traditional reboot, it revealed that the new storyline existed in a separate timeline, with the original series and movies remaining part of the main canon.
The return of Leonard Nimoy as Spock Prime alongside Zachary Quinto's new version of the legendary Vulcan confirmed this.
The show's placement in both timelines is a topic of discussion, with the original series and movies remaining intact in the Prime Universe.
Author's summary: Exploring Star Trek: Enterprise's timeline.