User blog comment:Subject 16*/Assassin's Creed: Revelations - News/@comment-121.54.48.42-20110514090854/@comment-1655340-20110514155408

Or, as someone brought out earlier, it could be similar to what happened to Altair in AC: As punishment for his actions in the beginning, Al Mualim stabs him with a fuax blade; it simulates the effects of a hidden blade stabbing, but didn't actually kill Altair. I don't know how they'd be able to explain that when Desmond clearly had an actual blade, but it's fodder for the cannon.