One-episode-stand characters: who's your fav one?

One-episode-stand characters: characters who were introduced in an episode, and then we never saw them again. Obviously, I don't mean people like Jacob or Norton's associate. The first is an example of a character anyway very important even if we saw him once, and the second is an example of a character we saw once, and we don't care seeing him again so much.
What I mean is... characters like Harper and Oldham. They were introducted with a few heating warm-up, they were supposed to be central in that single angle, but at the end of the 42th minutes of the episode... they suddenly disappeared.

Have you any other examples? And what's your thoughts?

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