What is this called? The trope where, in some episode, a character suddenly has advanced, expert knowledge in a field that they have never mentioned, especially something almost out of character for them, and especially it is never mentioned again in the series.
The one I just saw that made me think of it: In Daria S02E02 "The Daria Hunter" Britney displays expert knowledge in guerrilla combat when they go paintballing, but she is more or less a stereotypical cheerleader ditz in every other episode.
Others:
Community S01E23 Modern Warfare, S02E23-24 A Fistfull of Paintballs, For a Few Paintballs More, S06e11 Modern Espionage, where the main characters are all pro-level paintballers, but do nothing paintball related otherwise in the series.
How I met your mother S02E15 Lucky Penny where Barney successfully runs a marathon, after having displayed no serious athletic ability other than having sex all the time, and did not ever again for the rest of the series.
Weaker examples:
House S04e15 House's Head, where Chase, a diagnostician and a surgeon, suddenly becomes an expert hypnotist (not that great of an example since it is still a medical skill)
Simpsons S04e02 A Streetcar Named Marge where we find out Flanders is incredibly muscular.