The release date for Star Wars: Battlefront was leaked ahead of an official announcement. The source of the leak, the official game website source code.
AllGamesBeta captured a screenshot of the sites META data showing the description including the release date of the 17th November 2015, the site appears to have been updated with the offending date removed.
The META description read
The latest news and updates for Star Wars Battlefront. Available November 17, 2015 for PlayStation 4, Xbox One, and PC.
Also, eagle eyed viewers of the Star Wars: The Force Awakens Official Teaser #2 will see that footage of Battlefront is shown in the closing seconds of the video in a thumbnail to the left.
Lead level designer, Dennis Brännvall confirmed that it’s in-game footage and not CGI via a Tweet
I’m going to answer a question in advance: Yes, that is actually what the game looks like.
— Dennis Brännvall (@DICE_FireWall) April 2, 2015