The latest update to Forza Horizon 3, as noticed by NeoGAF, is a whopping 53GB download. That's an awful lot of data for a single update. As it turns out, that's because instead of patching the game to support today's Rockstar Car Pack, the update pushed an unencrypted developer build of the entire game.
The leak revealed a huge list of cars, many of them not yet added to the game—spoiler warning, but you can see the full list on Imgur—and may well have exposed the game to heightened risks of piracy as well. But the real problem for players, as explained in this forum post, is that those who downloaded the update, and then played and saved the game, will be stuck with corrupted saves.
"Avoid creating a new saved game on .37.2, and only play on .35.2 to avoid this issue," Forza community manager Brian Ekberg. "As long as you have an existing save and have not created a new one on .37.2, your saved game will work correctly once the update is available."
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.