It turns out an employee at Playground Games accidentally "greenlighted the wrong update file," pushing the internal developer build live to Microsoft's servers. According to gaming preservation and leak site Hidden Palace, this specific build contained "unreleased stuff" including the Blizzard Mountain DLC and various car packs that were not scheduled to be available yet.
In the context of Forza Horizon 3, the retail game files were encrypted to prevent tampering and piracy. However, the accidentally released version—specifically —was an unencrypted "Open World" development build. This meant that anyone who downloaded it could essentially open the hood of the game and look at the engine. forza horizon 3 dev build download
Because the game can no longer be downloaded through official Microsoft channels, any download links found today are hosted on third-party file-sharing sites. It turns out an employee at Playground Games
Even if you manage to obtain a clean version of the dev build, you risk corrupting your legitimate Forza Horizon 3 save files. As Turn 10 warned back in 2017, starting a new save on the developer build can permanently damage your saved data. Even if you manage to obtain a clean
While the dev build is played offline, Microsoft’s telemetry can sometimes detect "tampered" files or impossible garage additions. If you accidentally connect to Xbox Live while using these files, you risk a permanent ban on your Gamertag. How the Community Uses It Today
In January 2017, an official PC update for Forza Horizon 3 went wrong [1, 2]. Instead of deploying the standard retail patch, the developers accidentally pushed an unencrypted, 53GB developer binary (version .37.2) to the Microsoft Store [1, 2].
Understanding the Forza Horizon 3 Developer Build: History, Impact, and Legacy