16 made for three fucking Mao eps in a row. Although, now that you mention it, I suppose one could count that as a single-episode screwup, as opposed to a mid-season multi-episode sidetrack.
Unfortunately, after that finished up, Suzaku immediately displaced Cornelia as Lelouch's primary focus. Between the Toudou jailbreak and Euphinator there was a long stretch where Lelouch made little significant progress with his plans, compared to the first half of the season when he was actually getting shit done. (This in addition to the Mao arc, which similarly kept him from moving things forward.)