At the end of the special, in a very subtle addition to the rest of the episode, Clavis and Mavis wake up to find that they're all alone, symbolically representing the true end of their era, as well as all of their castmates. They say their infamous line and slowly walk away. A major Tear Jerker for long time fans.
Hell, the entire damn thing. For Nickelodeon, a network notoriously bad at remembering and accepting their own history and dead-set on ignoring anyone who has outgrown their shows (at least until recently) brought back five members of the original cast who no one in the show's target age group could be expected to remember, reused old footage, and gave us some truly amazing mash-ups. It also makes for a sweet passing the torch moment, given that the special added three new cast members and the resurrection of Vital Information.