The Brady Bunch: Several episodes where Jan's artistic talents and showing interest in stepfather Mike's work as an architect. These eventually play out in Jan becoming an architect and a partner at the Brady patriarch's firm.
Traded in Season 4's "Career Fever" – Greg, writing an essay for composition class, hastily states he wants to become an architect, while Jan (along with Peter) express interest in being in the medical field. By the time The Brady Brides rolls around, Jan is immersed in architecture while Greg is finishing his residency as a doctor.
Jericho has Robert teach his daughter to shoot to better defend herself from the uncertainties in their After the End world. Not only does it bring this estranged father and daughter together, but six episodes later she saves his life.
iCarly: In Spencer's 3-day stay in law school, he managed to learn some of the most basic knowledge in law also with the help of his law book. His "background" becomes very handy whenever the gang encounters legal matters like in iPromote Techfoots and iOwe You.
Sam's unerring ability to pick locks (as much as using a sharpened duck bone) has been of great help to Carly and Freddie (or assault Freddie from his apartment) in a lot of episodes.
In The Vampire Diaries, Elena spends some time lifting weights and training so that she can protect herself against vampires.
Katherine has been sipping vervain over hundreds of years to build up a resistance.
The Sarah Jane Adventures Alan is established as a former Skateboard champ in the first part of one story, guess how he KOs an alien in the second part? It was even joked about as being "Chekhov's Skateboard" on Outpost Gallifrey.
In the MST3K episode "Bloodlust", there is an early scene where the heroine goofily tackles the hero (for no apparent reason) causing him to jokingly remark that he shouldn't have gotten involved with "the daughter of a judo expert." Sure enough, the movie later contains a scene where the heroine uses her judo skills to dump a mook into a vat of acid.
In one episode of Monk, Randy spends most of his time playing jenga. In the showdown at the end of the episode, he uses these skills to knock out the bad guy using a stack of lumber.
In another episode, we find out Adrian met his wife when she wrote her phone number on a piece of paper on his back, for another guy. Natalie refers to this as a really useless superpower. Later in the episode he's able to find out where someone is staying as she wrote it on his back at the beginning of the episode.
The Wire: Snoop and Chris teach Michael how to pull off murders, which allows him to figure out when * he* is being set up for one and turn the tables.
In an episode of Sister Sister, Lisa claims that she has a sixth sense, citing that she once saw a ghost of a relative. This is played off as a joke, Laugh Track and all. Later in the episode, she meets the ghost of Ray's old wife.
Not really a "skill" per se, but the fact that Sam Tyler was a DCI in 2007 while a DI (one rank below DCI) after going back to 1973 becomes very important in the final episode of Life On Mars.
In one of the first episodes of Power Rangers RPM, Ziggy mentions that he is good with shadow puppets, something treated as a joke. This skill manages to save the Rangers' lives at least once or twice over the course of the series.
In Season 2 of LOST, Locke teaches Michael how to use a gun. Big mistake.
Angelus shows that he is really, really skilled at sketching, but it's not really commented on. Two years later, this is very plot important for the whole season as he has to discover what the visions from the PTB are by sketching them from descriptions.
In the Buffy episode "Earshot" when Buffy's ability to hear thoughts leads to her finding out something is planning a mass murder at school the next day.
In season 3 of Buffy, Willow and Buffy joke a few times about how Willow's witchcraft has only progressed to the point of levitating pencils. This later turns out to be a very useful skill in a world where vampires are intensely weak to pointed wooden objects. Willow's magic eventually becomes a focal point of the show.
To some small degree Xander's experience as a construction worker. In the final battle of the Season 5 of Buffy it allows him to attack the Nigh InvulnerableBig Bad with a wrecking ball. Just a distraction to be sure, but an awesome one.
Averted in a season 4 Buffy episode. Somebody from the main cast is turned into a demon who can sneeze instant cement; this power is mentioned, twice, but never used.
Giles' skill with black magic in Season 8 of Buffy.
Wesley has this. Specifically, darts.
In Torchwood, Gwen's husband is a truck driver. In the miniseries Children of Earth, he uses his knowledge of trucks (his company in particular) to save himself and Gwen at a critical moment.
It's useful before that as once he learns about Torchwood in the episode "Meat", he uses the truck to sneak Torchwood into the factory hiding a suffering creature the workers are harvesting meat from while it's still alive. It's also useful in the second miniseries Torchwood: Miracle Day when he and Gwen save Gwen's father from the overspill camps.
The basis of the show Psych, in which almost every episode begins with Shawn being taught a skill by his dad when he was a kid, only to flash-forward to the future and see it come in handy.
Subverted in an episode where Shawn and Gus's Three-Legged Race "skills" are demonstrated, and when it comes up in the episode...they trip on a stump and fall down a hill.
In Highlander: The Series, newly immortal Richie makes an enemy of an female immortal who was friends with Duncan. Duncan trains him to counter a specific move she likes to use. Naturally, he's able to win the fight using it, but chooses not to take her head.
The Jonathan Creek episode "The Three Gamblers" opens with Jonathan's birthday, and Maddie has given him a book called Cards As Weapons by one of his magic heroes, Ricky Jay. Later in the episode he's practicing the tricks in the book. At the climax he uses a well-thrown playing card to save Maddie's life.
In Leverage's "The Bottle Job," we see a flashback to Nate's dad breaking a guy's fingers while young Nate watched. At the end of the episode, Nate breaks Doyle's fingers in the same way to provide a little extra emphasis to telling Doyle to get out of town and never come back.
In the Merlin series (the one with Sam Neill) and novelization, Merlin uses his magic this way. At the end of the film, he restores the youth of himself and Nimue using the same Hand-Wizardry that didn't work while Mab was alive. The novelization goes further, by elaborating that Merlin has the power to alter reality by combining fairy illusion with human feeling, something only Half Human Hybrids can do, a trick he only uses once early on but later uses in the final book of the trilogy.
The Gates: To break the ice when trying to talk to his son in "Identity Crisis", Nick performs an old sleight of hand trick, making a coin disappear from his hand. Later, he uses the same trick to hide a flash drive containing info on the identities of ~10,000 vampires.
In Doctor Who, Donna Noble's claim that she is 'the best temp in Chiswick' seems somewhat irrelevant, but comes in handy on several occasions: in realizing that not one member of the staff at ATMOS had ever taken a single day of sick leave, and they were in fact hypnotized drones, in noticing that the numbers throughout the underground of Messaline were datestamps, and in operating Davros' typewriter-like death ship control panel, for example.
The last one wasn't a Chekhov's Skill it was due to the Deus ex Machina of her getting Time Lord intelligence.
In "The Great Game," in Sherlock, Sherlock showcases his ability to judge fluctuations in weight to the half-pound, by commenting on Molly's three pound weight gain (though she claims it's only two and a half). A similar skill helped him deduce Irene Adler's exact measurements in "The Scandal in Belgravia."
Inverted in the same episode: John casually mentions at the beginning of the episode, to everyone's amusement, that Sherlock has no idea the earth revolves around the sun; near the episode's end Sherlock almost fails to solve a case in time because of his lack of knowledge about the solar system. John lampshades this nicely.
The Master's ability to stop his heart and thus fake his death fools the baddie of the week. Though when he later explains it actually COULD kill him if he overuses it, one wonders why he wasted one of his attempts screwing with Max's head in the hotel at the start of the episode.
Max being forced to work on his cardio is rather useful when he needs to cross town by foot when he loses the van.
Parodied in the pilot episode of Arrested Development: one of Buster Bluth several graduate degrees is stated to be in cartography. When the SEC close in on the family while they are on their yacht, we get this exchange:
Lindsay: You’ve had $80,000 worth of cartography lessons. Get us a channel to the ocean.
Buster: Okay, okay, okay. [beat] Obviously this blue part here is the land.
In Three's Company, Jack complains about how their old roommate Chrissy's way of opening the door forcefully kept causing injuries, and attempts to teach her the right way. Then at the end of the episode, Jack, Mr. Furley, Janet, and the new girl were held at gunpoint by a psychopath who robbed the store earlier (the newspaper claimed that Jack saw the robber's face). After they knocked out the cop detailed to protect them, Chrissy bursts in and knocks out the robber. Jack asks her if she knows what she's done and she apologizes.
On NCIS Tim McGee's second career as a bestselling author (under a pseudonym) is usually the subject for jokes once found out, but in one episode when they need to get someone into an exclusive club, Gibbs realizes they don't need work to get in; they've already got a celebrity who would be invited past the velvet rope. To enhance the effect McGee arrives in the expensive sports car he'd earlier mentioned he'd just bought with Ziva and Abbie hanging off his arms.
In one episode of MacGyver, Mac is learning martial arts at the beginning of the episode, and near the end of the same episode uses the moves he learned to get out of a scrape.
In the first half of season 7 of Criminal Minds JJ takes down a large male Unsub. Afterward, Morgan teases her about her martial arts skills, and we learn that he has been training her. In the season finale, she uses the same martial arts skills to fight a trained female killer who has JJ's family.
Established in the premier of Spartacus War of the Damned. Crassus is receiving training from a gladiator, the gladiator disarms him and closes in for the kill only for Crassus to use a Bare-Handed Blade Block to wrench the sword away and stab his trainer. Sure enough, when Spartacus and Crassus face off in the finale Crassus (again after being disarmed) uses this on Spartacus, who uses it right back.
JAG: In the Pilot Movie, the fact that Harm is a former aviator who used to fly F-14 Tomcats is what enables him to save the CAG at the end.
The captain of the "Gimli Glider" happened to be an experienced glider pilot and pulled off some gliding maneuvers to land the plane after it ran out of fuel.
One of the pilots in "Fight for your Life" used to fly jet fighters in the Navy, and used his instincts from that area to maneuver his plane into taking a hijacker off-balance.
In the Supernatural episode "All Hell Breaks Loose, Part One" (S02, Ep21), Andy reveals he has developed the ability to project images into other people's mind to the other Special Children at the start of the recording by talking about beaming gay porn into the mind of a guy he does not like. Later this ability becomes important in giving them a chance at a rescue.