In fictional bars, attractive women don't normally talk to men who are less than absolute studs. They certainly never flirt with or outright hit on such men...so when they do, something must be up. This is when a beautiful woman approaches a less-good-looking man in a bar because she's coldly trying to lead the poor guy to his doom or take advantage of him in a non-sexual manner with her tricky feminine wiles.
The poor schlub will likely be Chained to a Bed, humiliated in some other fashion, short a kidney, or dead once the lady in question has what she wants from him.
See also Honey Trap.
Played with in Sin City: Goldie hits on Marv, but she's the one who ends up dead. She wasn't killed by Marv, though; she'd seen something someone didn't want her to see, leading her to hit on "the biggest, meanest lug around" for protection. Unfortunately, Marv got drunk and was passed out when what she was running from caught up with her. Cue Roaring Rampage of Revenge.
Mystique and a prison guard from X2: X-Men United. Sure, she drugged and injected him with iron, so that Magneto could rip it from his blood and escape, but he got to make out with Rebecca Romijn. Lucky bastard. This trope is basically lampshaded when Magneto tells the guard that he should never trust a beautiful woman — especially one who's interested in him.
In The Living Daylights, a humorous scene involves a mannish, overweight woman with large breasts seducing her boss to distract him from Bond's actions, by shoving his entire head inside said voluminous bosom. After Bond leaves, she throws him back and snaps, "What kind of girl do you think I am?!"
A genderswapped version appears in the 2009 movie Duplicity—hottie Clive Owen chats up a frumpy woman as a ploy to get her to take him back to her office in order to access the company's computer network. His female colleague is not happy about this tactic, despite the fact that she pulled the same stunt in their first meeting, in order to steal some files Owen was carrying.
In Sneakers, the group of shady security experts arrange a date between an employee of the Big Bad (a geeky computer expert) and their leader's ex through an early dating site in order to keep the employee distracted and out of his office while they break into it. The second the Big Bad hears of this he incredulously says "A computer matched him with her?" and locks down the building, knowing the good guys are up to something.
Enchanted: Narissa on Nathaniel. Nathaniel begins to realize he's a dupe when he sees a soap opera variant.
Space Mutiny: Lea in a Squick moment. Although Lea's attractiveness is somewhat informed. The riffing, naturally, lampshades the hell out of this: "You know, the last eight times this happened, the woman just wanted to get away!"
This is roughly half the plot of High School Big Shot; the popular Betty seduces bookish anti-hero Marv to get him to write an essay for her; the plan backfires and costs Marv his chance at a scholarship. This pushes him over the Despair Event Horizon, driving him to plan to steal a million dollars of Mob money kept at the warehouse he works part-time at. All to get Betty's attention. It doesn't end well.
Seen in Angels Revenge. Especially egregious as they're dressed in bikinis on a ridiculously cold, foggy day on a deserted beach.
Directly lampshaded by a girl in She's The Man, who tells a guy, "Girls with asses like mine don't talk to boys with faces like yours." Also inverted in that she gets dumped not only by her boyfriend, but by herboyfriend's sister.
Done by the mermaids in Pirates of the Caribbean: On Stranger Tides to some unfortunate sailors. Not only do they kill them, they eat them.
Notably, though, the sailors were (at least in part) trying to do the same to the mermaids, and they needed to capture one for reasons of their own.
Done in the horror film Trick 'r Treat by a pack of female werewolves.
The Best-Case Survival Handbook (a parody, of course, of the Worst-Case Survival Handbook series) has an entry on (paraphrased, since I don't have it on me) "What to do if you are approached by a fantastically beautiful woman who invites you back to her hotel room for a night of athletic, no-strings-attached sex". It goes something like "Run. The only person this happens to is James Bond and she always puts a scorpion under the pillow or something."
The second book of the Sten series invokes this when the beautiful younger sister of a major political player comes on to Sten. He recalls a training manual which stated:
"When approached on a sexual level, covert operators should remember that they have not necessarily been found attractive beyond the moon and the stars but rather that the person making the approach is allied with the opposition and attempting to subvert, to maneuver into a life-threatening situation, or to provide the opposition with blackmail material. In any event, until a life-threatening situation occurs, it is recommended that operatives pretend to be seducible. Interesting intelligence has been produced in such situations."
This isn't quite a straight example, however, as Sten is noted to be attractive enough that women often do approach him just for sex or romance.
Furthermore, it turns out to be a subversion. Her brother told her to seduce Sten, but she's not interested in helping her jerkass brother, and is genuinely interested in Sten.
A short story by Spider Robinson involved a painfully-awkward and very unattractive computer geek being approached by a memetic sex goddess. She puts a crown on his head, then takes him back to his apartment for all sorts of sex, saying, along the way, "Hurry, my lord, I'm positively dripping", while other bar patrons stare at him. After lots of awesome sex, he ends up figuring out that the crown is a recording device for a futuristic version of amateur porn. When you can experience the other person's point of view, a porn star is just too analytical to get the voyeuristic juices flowing.
Farscape inverts the trope. In "Fetal Attraction", handsome John Critchton hits on a much older Sebacean nurse with access to Aeryn. However she's suspicious and only relents when he throws in a cure for a disease running rampant on the station as well. That being said, he played the horndog well enough that, when he's captured, he's able to bluff a species that is mildly psychic that the reason he took Aeryn is because she was prettier than the nurse and he was planning to have sex and babies with her...which was technically the truth, but not the whole truth.
No result as serious as death, but when Veronica flirts with Phil in Better Off Ted, you know that she has ulterior motives. Turns out that the "autograph" she wanted was on a waiver promising not to sue the company.
Played with on The Shield. The Strike Team is tasked with investigating a series of robberies at a strip club—the girls proposition men for alley sex, but one of the girls has an accomplice who attacks the johns. Vic tries it, picks "incorrectly", and gets a blow job. Shane tries it and gets a tire iron to the head.
Sydney Bristow in Alias did this to bad guys in nightclubs all the time.
The titular character from Castle got seduced by an actress who wants a part in his movie. When he confronts her with the possibility, she breaks down in tears and asks "How could you? Do you know how hard it is for a woman in Hollywood?", leaving him feeling bad. Of course, she was just after a part. He ended up not rescinding his recommendation because she was a good enough actress to fool him into thinking he'd really hurt her with the accusation.
White Collar has a gender-flipped version, where Neal Caffrey seduces a woman in a bar to find secret information on his boss. (Of course, she's not quite as unattractive as male examples usually are.) Similar tactics to this are a normal part of Neal's arsenal.
Sarah Walker in Chuck once seduced a geek who had developed a secret weapon. Of course, she also attempted this with Chuck, although not in a bar, but in the Buy More, so the CIA could get their hands on the Intersect.
Fiona from Burn Notice does this many times, but it's also subverted on a few occasions. For example, in one case she approaches a professional safe cracker with multiple arrests for drunk driving and tries to get him liquored up so he'll be arrested again and his team will be forced to call off the job they're pulling. Due to the fact that Fiona is way, way out of his league, comes out of nowhere and then starts proposing toasts to them with expensive champagne, he figures her for a High-Class Call Girl and leaves.
Another time it works too well. Fiona tries to drug the guy with chloral hydrate in a martini, but he's more interested in her than the martini. So she switches to a different approach.
In Leverage, Parker does this to a mark in "The Fifteen Minutes Job" in order to sell the con (no one would look twice at him if he hadn't gotten some minor media attention). She didn't even stab him!
Sophie also does this somewhat frequently, though in her case it is usually somewhat justified in that her marks are often rather wealthy such that someone with her looks would logically be interested in them due to their success. It also doesn't hurt that as a sophisticated grifter she comes across as having class which also helps sell things.
This came up in CSI: Crime Scene Investigation as early as the pilot. A woman was seducing men from out of town, then knocking them out with a sedative applied to her nipple. She then robbed them blind.
In the first episode of Fresh Meat, Howard warns Kingsley about this when he pulls a beautiful blonde girl on their first night at the pub ("This does not happen! It's a trap! When you wake up missing a kidney, don't say I didn't warn you.") Turns out she's not an organ thief...she's an evangelical Christian, who talks Kingsley into setting up a standing order.
Lampshaded in Between The Lines. Mo is infiltrating an Animal Wrongs Group, and tells them she's going to seduce a security guard working at the animal testing center they plan to attack. The security guard is actually Naylor, also working undercover. When someone asks why she didn't pick someone younger and better looking, Mo replies that men are like dog shit. "The older they are, the easier to handle." Naylor however is embarrassed as Mo is both his colleague and a lesbian. Eventually Mo has to tell him to start pawing her as the others are watching and "I'm supposed to be seducing you, not taking you by force."
A pretty lawyer does this to get dirt on a witness in Boston Legal. Given that he's overweight and nerdy, he's immediately suspects that she's an escort girl, but she convinces him she just wants some casual conversation with someone who doesn't think he's The Casanova.
Certain perks in Fallout 3 and Fallout: New Vegas allow a player of either gender to do this by bypassing skill checks or bribery options to seduce NPCs into giving you information or otherwise helping you out. Or, rather hilariously, convince a man he is NOT a zombie. Long story.
In The 7th Guest, Martine Burden commits adultery with the married elderly man Edward Knox to secure him as an ally against Stauf Manor's other guests. Both end up dead for their trouble, just like all the others.
Played with in Spinnerette: Sahira attempts to seduce a guard named Buzz Rickards in order to allow Heather time to break into a lab, only for him to see straight through her and lampshade this very trope...then give Sahira the video the girls were after anyway, leading to the two dating.
In The Critic, Jay falls in love with a beautiful actress who swears to everyone that she genuinely loves Jay and isn't trying to butter him up for a good review. When he screens her film and discovers that her acting is God-awful, he decides to be honest and give an impartial critique of how terrible the movie is. Unsurprisingly, she immediately turns on him, giving away that indeed she was just using him to get good press (and actually was a great actress when it came to tricking Jay). Of course, being a schlub of the highest (lowest?) order, Jay still tries to pursue the relationship even after finding out it was a sham.
Averted by Jeremy Hawke's sister, who's a hottie and is genuinely into Jay, but it's because she happens to be attracted to needy losers. Her last boyfriend before Jay was Larry "Bud" Melman. After Jay? A hayseed who needed his pants zipper drawn—while wearing the pants backwards.
Played pretty straight in another episode where Jay is seduced by the woman working the projector. She has the sexy eyes that lure him to the booth for some loving, but she turns out to be nuts: kidnapping him so he can personally review every movie for her (she was sick of being embarrassed in front of friends by her inability to distinguish the quality of films).
In The Simpsons episode "Dumbbell Indemnity", an attractive woman at a club invites Moe to join her for a drink. Noticing that she mentions Bacardi rum in every sentence, he asks her whether she works for Bacardi. "No. I'm in love with you," she says, then affixes a "Drink Rum" sticker to his forehead and walks off.