Ubisoft is working on a fix
The new Hidden Secrets downloadable content for Assassin's Creed III has an issue which can cause players to lose their in-game progress. Users have reported the nasty bug on the Ubisoft forums and, thankfully, the company has reproduced the problem and is working to address it.
"From what we can tell, this problem seems to be rare and should generally only affect a very small number of players," wrote a representative. "That said, we're taking the speedy and complete resolution of this very seriously."
Ubisoft did have some advice for how to avoid this mess: "Do not load your saved AC3 game while the DLC is downloading in the background. This appears to be the key trigger for this bug. You can do anything else you want to do, just don't load up your saved game and play it while the DLC is downloading." Better safe than sorry.