Naughty Dog's The Last of Us contains a pretty awful autosave bug, demonstrated in the YouTube video above. If, at any time, you see "Autosaving. Please wait..." appear four times in the pause menu (and it doesn't get replaced with a confirmation message of when the game last saved) then this means your game has not saved since the last time it worked correctly. You cannot leave the game, save/load the game or restart a checkpoint.
I lost approximately 2 and a half hours game time this way and since then I had it happen again after only 10 minutes of play time.
It's exceptionally frustrating to find this kind of bug in a game you've only just bought, but there is a work around: do not sign in to PSN or play offline. The game seems to function as usual in that case.
Hopefully, it won't be too long before ND patch the hell out of it.