Forza Horizon 3 has been out a few months now and has had very little issues at all for a game with online implementation like it has, which is definitely pretty impressive. The game released for both Xbox One and Windows 10 PC, the latter of which just got a new patch that could lead to you completely losing your existing game saves if you play the game with this patch.
As revealed on the official Forza forums by a member of the development team, an incorrect patch was sent out for the game that first of all was making many players have to re-download the game upon trying to start it up. At that time, they just said to go ahead and cancel this out, because you would have to download it all again once they straightened it out.
This incorrect version of Forza Horizon 3 should end with version number .37.2, while the correct one that has now been put out is .35.2. If you have a version number ending with .37.2, make sure to uninstall the game and reinstall, which should have the version number ending in .35.2. However, there is a critical issue that was also addressed where they said:
“- NOTE: PC players who completed the download of .37.2 and then started a new game save will have a corrupted saved game. Avoid creating a new saved game on .37.2, and only play on .35.2 to avoid this issue. 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.”
This is obviously very bad, as you most certainly would not like to lose out on your likely lengthy game save at this point. As a result, if you have the game for PC, make sure to follow these directions, while also checking out the Forza post linked above for any other updates to make sure everything is good to go.