9. Grand Theft Auto - Red Dead Redemption
There's always been a modicum of debate as to whether or not the universes of Grand Theft Auto and Red Dead Redemption are one and the same. It's certainly true that GTA shares the same world as Manhunt and Bully, but the jury has always been out as to whether the exploits are John Marston are part of GTA's bloody history.
Well for those who are wondering, there's one little clue that may confirm this.
More easily noticed when roaming around in first person, several bookshelves littered throughout Los Santos are home to 'Red Dead' by J. Marston. A very much on the nose reference to Rockstar's other flagship series.
The existence of this book implies that after the events of Red Dead Redemption, Jack Marston became a writer, and given that the book is entitled Red Dead, you can assume the story is based on the exploits of his father, or it might detail what life was like for Jack after he got his revenge on Edgar Ross.
The only discrepancy between the two worlds is that whilst the Red Dead series uses actual names of places such as New York, the GTA series has always opted for fictional ones.
Still though, it's pretty hard evidence.