Konami has put out a public service announcement of sorts: there is a game-breaking bug in Metal Gear Solid V. It's completely avoidable, but it exists.
Konami is currently working on a fix, but until then there is a surefire way to avoid it.
Simply put: if you use Quiet as your 'buddy' in missions 29 or 42, there's a chance that your save file will be corrupted. That is bad news, particularly that far into the game. My advice: even if Konami does find a way to fix this bug, I'm playing it safe. Forever. No way I'm using Quiet on that mission whether there's a patch or not.
But there will be a patch — that is currently being worked on — but better safe than sorry for now.
Via Game Informer