It's not uncommon for video game companies to reference their own games in...their own games. But Yakuza 5, never one to turn down a tie-in, is something else: Yakuza 5 features another company's game.
Here, Namco's Taiko no Tatsujin appears in Sega's upcoming gangster opus. When you visit an arcade in Yakuza 5, you can play the Namco rhythm game.
Sure, Japanese game companies often loan characters to other franchises -- whether that's Monster Hunter appearing in Metal Gear Solid or Sonic and Mario doing video games together.
However, this is one developer's game appearing in another developer's game. I'm sure there are previous examples of this -- or examples in other examples.