The latest "Forza Horizon 3" update, as observed by NeoGAF, is a whopping 53-gigabyte in size. Yes, you read it right. What actually happened was instead of updating the game to support the "Rockstar Car Pack", the update sent an unencrypted developer build of the entire game.

The leaked patch revealed a big list of cars, most of which are not yet a part of the game. The full list in Imgur might be a spoiler though and also a possibility that the game may be pirated after the incident. The actual challenge for players, explained in a forum post, is that those who downloaded the patch and saved will have corrupted saves.

Brian Ekberg, Forza Community Manager, advised avoiding creating new saved games on .37.2 and only play on .35.2 to avoid the stated issue. He also said that as long as you have an existing save and gave not created a new one on .37.2, the saved game will work properly once the proper update is available.

The unencrypted developer game version that was supposed to ba an update patch was already been pulled, but in case you have already downloaded it and played the game or made a save, you will have to uninstall the game and redownload the full game. The wrong version is ending in .37.2, while the proper, playable build will end in .35.2. The version number can be seen on the launch screen of Forza Horizon 3. We strongly advise that you check it first before playing the game or you might have to redownload it all.

The update has now been pulled, but if you've already installed it you'll need to uninstall the game and then redownload the whole thing. The incorrect version will end in .37.2, while the correct, playable build will end in .35.2. The version number can be found on the Forza Horizon 3 launch screen; check it before you drive, just to be sure.