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.
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.
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.