Anyanka was introduced as a one-shot character but was brought back for Buffy the Vampire Slayer: Doppelgangland where she was made more humorous. When the producers realized how funny Emma Caulfield was they simply kept bringing her back, until she was made a regular cast member (including appearing in the opening credits) at the start of season five.
When Buffy-from-Cleveland says, "I don't play well with others," this is the ultimate proof that Cordelia's wish really brought us into another universe, although some have tried to argue otherwise.