The lawn mower scene... (Spoilers)
The scene from "Guy Walks Into An Advertising Agency" where Lois massacres the British executive's foot with the lawn mower is one of the best-directed moments in TV history.
Every little detail of the scene is so meticulously thought out and brilliantly executed: the way the atmosphere changes from casual to chaotic in a few seconds, people looking shocked or about to vomit, the tragic irony that nobody knows what to do except Joan, who has just been thrown a farewell party and is about to leave the company, and Pete being the one who catches Peggy when she faints. Not only is the scene itself very well done, but all the incidents that precede it slowly and quietly build up a sense of impending doom leading up to that bloody accident: the male employees talking indifferently about the violence of the war in Vietnam, Joan and Peggy being interrupted by the noise just as they have finally been able to admit their mutual respect for each other, and Lois having been so incompetent and unprofessional so many times in the past that the moment you see her jump on that lawn mower machine you know something bad is going to happen.