Game Breaking Assassin's Creed 3 bug

I was doing one of the Peg Leg missions (after I beat the main quest if it's important). I ended up getting caught when trying to assassinate the two guards at the bottom. When it reloaded, I ended up getting the bug you see in the video. It happens even when I go down the stairs. It persists with a 'load from checkpoint' and even when exiting the game and re-starting it! Has anyone else seen this bug? What causes it and how do I fix it? Edit: as so often happens, after I posted this video, the glitch was gone when I reloaded the game.Very odd indeed.