Despite having been out for a little while now, over the past 24 hours we've received a number of complaints from users saying that Thief has suddenly developed a nasty game-breaking bug.
Likely the result of a recent update, the bug is stopping people from progressing past Chapter 4 in the game.
We contacted Eidos and were told the following:
We're aware of an issue that seems to be affecting some Thief players, preventing them from completing some levels. We're taking this very seriously -- as a first step we're reading as many reports of the issue as possible, working to replicate it internally, so that we can understand what's causing it. We're sorry for any disruption to your game today, and we'll update you as soon as we have more information.
If you've just got the game, you might want to hold off getting to the end of Chapter 4, at least until there's a fix.