1. The lawn-mower incident. From “Guy Walks into an Advertising Agency,” season 3 episode 6, Sept. 20, 2009.

This was a stunner: Lois boards the John Deere that someone has procured to celebrate Ken's landing the Deere account, and the secretary runs over the foot of one of the new London bosses, Guy MacKendrick. The brief moment of gore let us know that anything could happen in the orderly "Mad Men" universe, signalling the sea changes to come. It also created opportunities for black humor, as Roger makes a joke about the episode and the other London boss makes an unintentional one, lamenting that Guy won't ever be able to play golf again.