Sony has said it is looking into an error that apparently corrupts PlayStation 4 save files. Users have been citing the "CE-34878-0" error, a crash error code, and finding some save data compromised. Sony has given a few workaround solutions to try while it looks into the problem.
If you're already experiencing the issue, you have a few methods to try rather than simply waiting for Sony. The support site suggests exiting the game and installing any pending system software or game patches. If that doesn't work, it recommends you back up your saved data, and then initialize your PS4. That option can be found in the Settings menu under Initialization. That's a drastic step, since it will wipe all settings and info stored on the system--hence the need to back it all up first.