Well, somehow it took me 120 hours (according to Steam), but I finally finished Deus Ex: Human Revolution this weekend.
I was EXTREMELY thorough. Every computer or security panel that could be hacked, I hacked. Regardless of whether I had the code or not. I also played Pacifist and avoided killing anyone my entire play-though (or so I had thought, see below). The benefit was that I had an insane amount of XP by the end. I was able to upgrade almost every single augmentation.
Long before I reached the 2nd boss I had already upgraded everything that I wanted to.
Once the credits started rolling, though, I realized that the Pacifist achievement never popped up. I was also expecting the Foxiest of the Hounds achievement as well, but that also never came.
Turns out that I killed some people during the prologue (before you get augmented or are given the option of a non-lethal weapon), so I lost the option within the first hour of playing.
I know in my heart that the moment I was given a tranquillizer gun and given the option to not kill people, that not a single human was injured by my hand. I was extremely careful and continuously reloaded from my save files whenever something went wrong. It just sucks that I lost the achievement before I even knew you could play non-lethal.