 The scene where Dog shows Alyx his plan to hurl them across a canyon  back into the Citadel before it self destructs had an unexpected  addition added to it that was not originally planned but kept in.  Originally, Alyx was supposed to tell Dog, "You did do the  math...right?" but during the development process of the scene a bug  was accidentally put in Dogs animation, and after Alyx delivered  his line Dog shook his head as if to say no. When play testers saw  this their reaction was hysterical laughter. People enjoyed it so  much the animation was kept in the final version of the game.