24 generally made this an Averted Trope, as character death would never really be announced it all to make things as shocking as possible, but it was used halfway through the fifth season, as the previews for "Day 5: 7:00 P.M. - 8:00 P.M." stated one character would sacrifice himself to save everyone. That turned out to be Lynn McGill, but if anything, the use of this trope was played in order to surprise audiences for the real intended major death in the same episode: Tony Almeida.
One episode advertised itself in this manner, and the death in the end turned out to be a character who was not only minor, but also evil, and had a completely different personality to that he had had in the previous series, for no explained reason.
It's explained later in the fourth season he was possessed by a person from the future between the previous season and the one where he died. This is actually a major plot point in season 4.
There is another episode where the trailer advertises the main characters being trapped together and two would not make it out alive. In this episode, Shawn Ferrell and Meghan Doyle both died, but because it was a dream (sort of), they were both fine at the end.
All Saints: In one episode, a few close-ups of stern faces were all that were needed to completely fool the audience into believing that "One of these women will leave the show... FOREVER" meant that one of the two feuding female characters would die, only for one of them to leave the show quietly, and furthermore, temporarily.
Arrested Development parodied this. Before the opening credits, the Narrator announces that "one of these characters will die," while every main character flashes on screen, as well as an old lady who is unique to the episode. However, about halfway through the episode, the Narrator simply tells the audience that she is the one who's going to die:
Episode 7 of season one had this. The Clown announces towards the beginning of the episode that "Something dreadful is going to happen. I'm going to take someone." This comes into play when Shaz is stabbed and is technically dead, before Alex revives her.
This becomes rather ridiculous in hindsight after seeing the end of the series when it is revealed everyone including Alex had been Dead All Along and existing in police purgatory. The only way the Clown makes sense now is as an alternate form taken by DCI Keats, both characters having tried to "take the soul" of one of the dead characters - Keats having succeeded.
Beverly Hills 90210 had a memorable example in the second season when we were told an original character would be killed off, and of course it turned out to be David Silver's friend Scott Scanlon.
Blue Heelers had a run with Gary Sweet playing a gangster. One promo hyped how there would be three shots fired in the next episode (actually five, four of them being used to execute two crooks), with the final shot being the most shocking. The final shot? A hit on Sweet's character.
Bones featured the murder of a minor regular (at the hands of the sniper the team is currently hunting). The build-up made a point of intercutting the killer with all regulars to make them look threatened. However, the actual victim is foreshadowed by the episode title. which is an allusion to the episode in which Zach left the show.
Breakout Kingssubverted this so many times that it became annoying. When the commercials for Season 2 began to air, this was the only thing they talked about. But the creators threw a curve ball and actually did kill off one of the main characters, proving to everyone that they do have the balls to have someone Killed Off for Real.
Buffy the Vampire Slayer used this in a promo during the second season. The clips made it abundantly clear it would be Jenny Calendar.
Castle pulls this for its third-season finale. The promo specifically has Castle saying "Someone is gonna die." The producers promised that it wouldn't be a background character. True to the formula, the line is actually only tangential, but the promise was kept: Captain Montgomery died while getting the bad guy. And then...
Leading up to the two-part finale of Season 6, it was heavily advertised that someone would be shot, and thus implied that a main character would die. In the end, the victim was Jim Brass, an oft-recurring but non-central character. And he survived.
Again in the 2010 spring finale: Nick gets shot by Dr. Jekyll, but not only does he survive but is also the one to kill the bad doctor. All seems well until "The Dick and Jane Killer" shanks Langston — and cut to credits.
Dawson's Creek used this to promote one episode in which minor character Abby died.
The finale for Series 2 was set up with a trailer showing Rose saying "This is the story of the day I died", and making much of the fact that actress Billie Piper would not be returning. Of course, she only died metaphorically, being sent to a parallel universe and thus separated from the Doctor.
It's explicitly mentioned in the episode that she was declared legally dead after disappearing during the Battle of Canary Wharf, so as far as the government's considered the claim was true.
In Series 4, the same trick was used with Donna Noble, who didn't actually die, but the person she had become "died" metaphorically due to Laser-Guided Amnesia.
In the specials following Series 4, one major arc was the upcoming death of the Tenth Doctor. Of course, being a Time Lord, he regenerated into Matt Smith. Despite the Tenth Doctor's angst about regenerating, some might consider a phoenix-like resurrection with a new face and slightly different quirks better than completely ceasing to exist.
In the Doctor's own words: "Even if I change, it feels like dying. Everything I am dies. Some new man goes sauntering away... and I'm dead."
And again in the buildup for Series 6 - coupled with a promise from Steven Moffat that "we're not lying, we're not cheating. One of those four people is going to die." Matt Smith's character dies (and doesn't regenerate this time) but as we find out in the series 6 finale, it wasn't really the Doctor.
Emmerdale: While not exactly outright said, it might as well have been advertised with how blatant it was. The character Tom King was killed off after just about everyone mentioned wanting him dead ("You do realise that at this precise moment, Dad's worth more to us dead than he is alive." "If you had died instead of Mum, we'd be a lot happier."). He even mentions it himself ("If I were to go now, I'd die a happy man.").
This trope was pulled with Robert Romano. However, since he was the Dr. Jerk, nobody inside the hospital noticed until the cops told them.
ER was a serial abuser of this trope, but did make it a Subverted Trope a few times...like killing off a major character (Gallant) in the teaser with no warning, letting the rest of the episode being the hospital's reaction at the news (as he was killed in Iraq).
Grey's Anatomy used this trope when, following the second part of a three part arc, the audience was told that someone would die. Considering the title character was lying on a hospital bed not breathing at this point, this was a fairly serious claim. Naturally, however, the minor recurring character died.
A clever example: After being told that someone would die, we saw Claire's mom collapse to the ground. After the commercial, we're now in the hospital with the possibility of brain cancer. Had this character died, it would have been quite dramatically valid. Then she got discharged, just in time for Isaac to accidentally shoot Simone dead.
The first season played this card with the question: Who will explode? Peter, Ted or Sylar. Several characters in the finale were shot (Matt Parkman, DL), stabbed (Sylar) or seemed to blow up in the Big Sky Explosion (Peter, Nathan), but EVERYONE re-appeared alive in the second season. DL did recover from the gunshot but was killed in between seasons.
Not to mention Season/Volume 2, with the much-hyped painted death prophecy for Bennet. He does indeed get shot through the eye. Maya also takes a bullet and dies. However, they're both revived with Magic Blood almost immediately, or by next episode, respectively.
Hollyoaks had a clever subversion: The viewers were promised that a character would die during trailers for a special hour-long episode, only for that character to still be alive and well at the end of the special. He was then killed off in the next regular episode.
In an advertising variant of Beam Me Up, Scotty!, Lost has become timelessly associated with this trope despite only using it twice—in season 1 when Boone died, and in season 2 when Shannon died. In fact, the show has a better reputation for suddenly killing off beloved main characters with no advertising fanfare whatsoever.
Played painfully straight with "Homecoming", an earlier episode in Season 1, which was hyped with the "Someone will die" tagline, only for them to kill off minor character Scott of "Scott and Steve" fame, who had little involvement in the plot and barely any lines in the few episodes he appeared in. The "someone" could have also referred to Ethan, who was killed at the end of the episode, but the trailer seemed to specifically indicate that one of the main characters (i.e. the good guys) would bite it.
This silly advertising technique became notorious in Australia after it was used for every single episode of Lost in Seven network advertising, usually in the form of "Tonight it will finally be revealed what's in the Box/Bushes/Hatch/Plane/Water/e.t.c" but also in the more true to trope "Someone will DIE".
The second season finale featured a plethora of tropes associated with Tonight Someone Dies, almost to the point of subversion. Ads for the show featured the requisite montage of major characters and promised one would not survive the finale. Throughout the episode, characters narrowly survived snakes, bombs, and gunfire, and kept talking and dreaming about each other's deaths. In the final minutes, Kate took a bullet but was revealed to be wearing a bulletproof vest (Disney Death) only to be shot between the eyes in the final seconds (Killed Off for Real). As this was precipitated by Sasha Alexander's decision to leave the show, it seemed to be an instance of Dropped a Bridge on Him (although she appeared for most of the third season premiere as a ghost/hallucination).
And they did it again in Season 5. This time the character that died was Jenny Shepard, at the end of the first half of the two-part finale. Shepard, however, died in a blaze of glory, taking down all of the bad guys with her. All the grizzled Badass, Mike Franks(whom Shepard had asked for help), wound up doing was finish off two of the badguys who were bleeding out.
And to add injury to insult, Mike Franks himself became the Someone Who Died near the end of Season 8.
The Australian Network 10 announced on every ad in the 3 weeks before the finale of this show's season 3, "Marissa is going to DIE." Yes, everyone knew Mischa Barton was leaving the show, but this advertising strategy completely undid the finale's setting up of Marissa going to live with her dad, a plausible explanation for the departing actor which would have made the death something of a shock.
The US promotions of the same episode also said, almost verbatim, Tonight Someone Dies. The faces flashed as possibilities were all long-time regulars, so the promotion did its job quite well.
Person of Interest: Promos for "The Crossing" indicated that a major character would die. It was Carter.
Reality shows frequently subvert this, where the trailers and "Next Time On..." clips will show emergency vehicles rushing to the set. Usually it turns out that the emergency service will feature as part of the task (such as in the episode of Hell's Kitchen USA where the task was to cook breakfast for the fire service, yet the trailers showed fire engines rushing to the restaurant interspersed with regular shots of flames from the kitchen's stoves).
Hell's Kitchen itself did play it relatively straight in one episode though, where a contestant was legitimately taken ill during filming. Didn't stop them playing up the emergency vehicles in the trailer though.
Reaper heavily promoted the season one finale as "Someone Will Die". However, it seems that this death is the main character's (Sam's) dad, buried alive and presumed dead by the characters. In the last few minutes Sam's mom seems to have dug the still-alive man out of the ground. In a later episode, it was relieved that the commercials were technically correct. Sam's dad is no longer alive. Apparently Exact Words matter when making a Deal with the Devil.
Taken Up to Eleven in the first season finale. Casualties include Anne's parents, Increase Mather, Isaac (the mallum's first victim), and possibly John Alden.
Shortland Street, after playing it straight through many previous deaths of major and minor characters, subverted this in 2008 when one episode ended with several of the characters seemingly falling victim to a shootout in the Cliffhanger. The On the Next promo immediately following the episode announced that there would be three casualties. Naturally many fans assumed that there would be three deaths, until the actual definition of casualties was pointed out. Sure enough nobody died.
The Sopranos, though not explicitly promised, had established a tradition of suddenly killing off a major character at the end of each season. Then along comes the series finale, and... Smash to Black in mid-scene.
There was a major Wham Episode in which a popular main character dies unexpectedly at the end of a comedic filler episode. This would have been a huge shock, except the promos for the episode played up the Tonight Someone Dies angle (which spoiled the fact that someone was going to die), gave away which character was going to die, and even showed the first few moments of their death. To top it off, while the character did manage to stay dead for a couple seasons, he eventually came back as a clone.
A similar plot twist occured in the Stargate SG-1 2-parter "Heroes", with a major character who had been with the show since the very beginning being Killed Off for Real in the middle of what starts out as a comedic filler episode. Although on that occurrence the promos were nice enough not to spoil the fact that a major character was going to die, although they did hint the episode would be much darker than the initial premise would suggest.
SG-1 had another problem Tonight Someone Dies episode when Daniel ascended - the ads were hyping that "One of them will die", but a) it didn't take and b) any surprise was ruined by the TV guide reporting that "Daniel is exposed to radiation".
Possibly a Justified Trope in the episode No Rest For The Wicked. It's the finale and Dean's year was up; how else do you expect them to advertise it?
This trope was also used in Australia to advertise the episode Mystery Spot, although to be fair saying that 'tonight someone dies' was a bit of an understatement really, in its own way. Plus, the whole thing ended up being subverted. Tonight Someone Dies... 104 times in a row.
At the end of Season 1, it was Artie. He got better thanks to an artifact.)
Used again for the penultimate episode of Season 2 "Buried". Recurring character Benedict Valda makes a Heroic Sacrifice. Doesn't get better.