Trigun: The Planet Gunsmoke (lost build of unfinished video game; 2002)
Trigun: The Planet Gunsmoke was a video game based on the Trigun manga series that was in development for the PlayStation 2 by Red Entertainment and was to be published by Sega. A 20-second teaser trailer for the game was shown at Sega's GameJam event on March 2002. However, the game was cancelled afterwards for unknown reasons, with Sega issuing a "no comment" when talking about it.
It is widely spculated that Red Entertainment's PlayStation 2 game Gungrave (which was released in Japan four months after that year's GameJam event) is actually a modified version of their Trigun game following it's apparent cancellation. Additionally, there are similarities between the Trigun universe and Gungrave that seemingly back theory up, although it has never been confirmed nor denied by any of the game's developers.
Whether this speculation is true or not, the original game or any prototypes have not surfaced anywhere, and it's unknown how much development was done on the game in the first place.