Up until the 1980s, it was common for US network drama series, particularly those of the crime, action or sci-fi genre, to eschew a pre-credits sequence or teaser for a "tonight on..." sequence showing highlights from the episode about to air. This often spoiled plot twists and even, on occasion, the ending of the episode!
FOX has a tendency to completely ruin the element of surprise on their gameshows, including Moment Of Truth and Are You Smarter Than a 5th Grader?, by having openings and "coming up after this commercial" previews which show how far along the contestant is going to get, which completely defeats the point of going to commercial after the contestant answers the question but before it is revealed if they are correct or not.
Likewise the "later tonight" promos in their Sunday night cartoon block tend to show the best punch lines from the forthcoming shows, which would've been funnier if you had seen them in context for the first time.
In the Season Four finale of So You Think You Can Dance, the show cut to commercial before announcing whether the winner was Joshua Allen or Stephen "Twitch" Bass. During the commercial break, a promo for FOX News at 9 advertised a story about "So You Think You Can Dance champion Joshua Allen." No prizes for guessing who was crowned the Season Four winner when the show resumed...
NBC did one worse: they hyped the first millionaire of Deal or No Deal about one week before the episode aired, and considering how desperate NBC was to get a millionaire, even a casual channel surfer would get it. (It doesn't help when you use phrases like "It's the one you've all been waiting for!")
NBC does a masterful job of subverting this trope, though, nearly every week on The Biggest Loser. During the part of the show when the contestants are participating in their weekly weigh-in, they love cutting to commercial a split second before revealing a contestant's amount of weight loss with the final shot being a close-up shot on the face of one of the trainers or another contestant as they react to the number revealed. When the show returns from commercial and reveals the number, the reactions are usually the complete opposite of what was implied by the reaction shot before the commercial.
Ru Pauls Drag Race (especially the Untucked program) has a bad habit of spoiling quite early into the season how far different contestants will make it, simply due to the fact that you know they're going to last at least another week because either one of the season promos, a trailer for the series, or the Spoiler Opening shows them wearing an outfit you haven't seen them in yet.
German TV stations are particularly notorious for this. A trailer for Evolution with David Duchovny featured one of the movie's final scenes, a trailer for season 2 of LOST featured the confrontation between Mr. Eko and the smoke monster, a trailer for season 4 reveals who the Oceanic Six (one of the "main" mysteries in the first half of the season) are and on top of that features scenes from the season finale (the Oceanic Six arriving at home, the island disappearing), and so on.
Nickelodeon is just plain terrible at making trailers. The hour-long special Big Time Concert showed various clips of the boys back in Minnesota, which implied that they had failed in some way and returned home. Worse than that were the clips of the guys reuniting happily, followed by a detailed sequence of them getting kidnapped by Hawk moments before their show, escaping via Carlos's... bravery, and performing at their concert. Let's hope the channel never picks up a mystery series, since every trailer would reveal the culprit-of-the-day.
Another notable example of this is the Christmas special. The first promo showed the weather forecast stating that the airports at Minnesota have been shut down due to a snowstorm, which happens less than four minutes from the end of the episode, then shows the boys and Kendall's family greeting Mr. Bitters on Christmas morning, which happens in the next scene after that.
Reality shows on Bravo typically show the judges' harsher comments and contestant reaction shots/defenses. Although this is sometimes subverted, like one time where a comment was said in the trailer and the contestant shot showed him tilting his head back and going down, as if in frustration/agony. In the episode, he was in the top 3.
Ads have started running for a TBS stand-up comedy show which says "If these are the punchlines, imagine what the set-ups are like!" and proceed to show just the punchlines of jokes. Which tends to ruin the joke, but apparently nobody at TBS knows that, or how jokes work.
Happens frequently on WWE programming in Canada on The Score when during commercial breaks they announce upcoming events in Canadian cities, and announcing the matches...even when the match may include a performer who was "fired" on that same episode.
Happens constantly in shows promoting upcoming events in the States as well; it's not at all uncommon to have a major showdown between Superstar A saying how he'll be trying to take the belt from Superstar B segue into a commercial break... Which includes an ad for "[Current Tour Name] is coming to YOUR TOWN! WWE Champion Superstar A tries to keep Superstar B from reclaiming the title!" Well gee at least they're planning ahead. Then again, "On live TV, anything can happen!"
The Score at least tries occasionally, making a comment such as a recently "fired" performer showing up "due to contract obligations" or some such excuse.
If you have a box set/are watching episodes of a TV series on YouTube of a show you've never seen, never, EVER read the episode summary, unless you want the whole episode spoiled for you. In one sentence, the summary will ruin your whole viewing experience.
The 10th Kingdom: NBC was notorious for doing this during most if not of all of their miniseries "events", but a particularly egregious example occurred twice for this show: just after the suspenseful scenes in which Virginia and Tony were trying to buy the Traveling mirror at auction, the trailer revealed that it gets broken, and right as we're wondering if the heroes will get to the ball and stop the Evil Queen's plot in time, the trailer revealed all of Wendell's guests collapsing from poison. Next commercial break then shows us boththe same guests awakening, revealing they weren't really dead and Prince and Wendell switching back—though granted, this was something of an Untwist by that late in the game. About the only major plot point not revealed by the trailers, thankfully, was that the Evil Queen was Virginia's mother.
Not to mention Wolf stopping the Huntsman from killing Virginia at the end.
24: This show is somewhat notorious for this among fans; there's a fairly large portion of the fanbase that refuses to watch the "On the Next" previews at the end of each episode. Since the show takes place in real time and is largely fixated on the suspense of "what happens next", it's easy for a preview to take the suspense out of quite a few minutes of the upcoming episode. Examples are really too numerous to list, but here's a fairly recent one:
At the end of an episode in season 7, the ambassador from Sangala had locked himself and his wife in a panic room. The bad guys are outside, wondering how they can get to him. Cue the preview for next week, which shows the room being flooded with gas. Naturally, it's no surprise next week when Jack Bauer (working undercover) suggests that he can create a gas out of household items and pump it through the ventilation system.
Done again a few episodes later when Tony warns Jack that a major attack is going to happen in Washington D.C., but he doesn't know exactly where. Previews for the next two episodes come up and we immediately see the White House being taken under siege.
Season 3 had a very notorious example. At this point in the season, there was a powdered form of a virus being transported in a plastic bag by a mule (just a kid who agreed to carry something over the Mexican border). CTU spent the entire episode trying to track down the package and the kid. Then, after the episode was over the preview literally had Jack Bauer yelling "THE VIRUS IS OUT!" in absolute panic. Granted, it turned out to be a false alarm but 24 fandom was so pissed off at potentially being spoiled that complaints were flown at FOX's direction and addressed. This resulted in the previews being treated as spoilers in 24 fandom discussion.
A trailer for an episode late in season 4 spoiled the return of David Palmer in the episode even though he was uncredited to keep his return in it a surprise.
However, a few episodes that have pulled the Our Hero Is Dead cliffhanger notably avert this by having the following trailers show no signs of Jack in them to keep his survival somewhat more suspenseful ([[Contractual Immortality for all the good they do, anyway). However, at the same time they do spoil some other key twists happening to some other characters within the episode.
Trailers for Agents Of Shield a new ABC 2013 TV series set in the Marvel Cinematic Universe about the secretive organization with a penchant for employing badasses, shows that Agent Coulson, heavily implied as dead in a scene of the film The Avengers, is alive and well and snarky as ever.
Arrested Development: Subverted Trope with its 'Next time on 'Arrested Development...' sections at the end of each episode, which depict events which are then never shown to occur in the next episode.
Making it even more surprising the one time it did happen: Tobias sneaking into the blind attorney's home.
Arrow: An Australian promo for this show that aired a few weeks before the show began screening there intentionally gave away the twist at the end of the pilot.
Battlestar Galactica (Reimagined): Happens pretty often in trailers for the 2000s show. The promo for Resurrection Ship, Part 2, apparently attempted to be discreet in its final frame, which showed a hand holding a gun, aimed at Admiral Cain's head. However, the shot of the hand was detailed enough that many astute viewers were able to determine that it was "Gina", the Number Six Cylon imprisoned on the Pegasus, well before the resolution aired.
Worse still, the opening Title Sequence (sometimes) contains cuts from the upcoming episode, frequently turning the opening into an automatic, hard-to-avoid spoiler.
In the trailer for Revelations, every scene but one has already occurred by the episode's apparent ending, and that one scene can literally be missed if the viewer blinked. Even when you see it, it's ambiguous.
The trailer for The Ties That Bind pretty much gives away the fact that Cally doesn't survive the episode, though how and why are still a mystery.
There's another trailer that makes the how and why very clear.
The Hub had an interesting case. The trailers showed the newly-ressurrected D'Anna telling Roslin that she is a Cylon. Many viewers wondered if it was real or creative editing. As it was revealed, D'Annadid say that to Roslin. However, she said to to mess with Roslin's head. In the commentaries of the episode, Ronald D. Moore expressed great anger that they ruined the joke by putting it in the trailer.
Not to mention showing D'Anna was a spoiler in itself.
This show had On the Next trailers that were notorious for this. The one for "Phases" revealed who the werewolf was, and the one for "Innocence" revealed that Angel had lost his soul.
The DVD booklet for the first season contains a quote that spoils the outcome of the season finale.
You know that guy who dies at the end of Buffy's seventh season? Well he shows up in the fifth season of Angel, in a shocking surprise at the end of the first episode. Oh, he's also featured prominently in the opening credits.
The The Big Bang Theoryepisode "The Hesitation Ramification showed Penny proposing to Leonard. Immediately the next day the show's Facebook page showed a picture of the scene, ruining it for everyone who plnned to watch the episode on rerun/DVR, and also to everyone from countries where the episode hadn't been emitted yet.
The Buffy season 6 DVD has Dark Willow on the cover, and, even more inexplicably, on the first disc.
TeenNick is about to start running Buffy repeats. Their ad announcing this shows Buffy and Spike kissing. OK, maybe that's It Was His Sled by now, but the commercial is aimed at people who are not familiar with the series, who will now probably be quite confused when they start watching season 2.
When Fox ran trailers for the season 5 finale it prominently featured Buffy's gravestone, giving away the ending. Worse yet, it was practically played on a loop, making it near-impossible for viewers not to get spoiled if they simply tuned into the channel.
As for Angel, the promo for the series finale fairly clearly shows Lorne shooting Lindsay.
Chuck: During the third season, one episode ended with the implied death of Devon Woodcomb, aka, Captain Awesome. However, almost immediately afterwards, we see him in the next time trailer, still alive.
Even worse, one episode ends with Chuck about to meet his father (who ran off years ago) in a trailer in the middle of nowhere. The episode ends with the door opening, and Chuck's father hidden. Immediately afterwards the next-episode trailer proclaimed "Next week on Chuck, Scott Bakula returns to NBC!"
Done again in the fifth season, with "Chuck Versus the Curse" ending with a jail cell about to open, and the scene abruptly cutting after that. Lo and behold NBC's next-episode trailer revealing that Daniel Shaw is the person who's about to exit the jail cell.
Degrassi: Does this often, especially from season 10 onward. The commercials also weirdly combine this with Trailers Always Lie in that they take dialogue out of context.
Doctor Who: The new show has a strange relationship with this; for the tenth and eleventh Doctors, even though sneak peeks are run at the end of most episodes, they make sure that any trailers for the second part of a two-parter are given as much warning as possible, moving them to the end of the credits and giving plenty of room for a continuity announcer to explain. Russell T Davies is a very vocal critic of spoiling trailers, and often directed editors to screw around with press copies.
Played straight with the first Cliff Hanger of the revived series; "Aliens Of London" ends with the Doctor being electrocuted to death by the Slitheen, which then immediately cuts to a On the Next trailer not only showing the Doctor very much alive but also telling the army about the Slitheen's plans!
The Daleks being the Big Bad was treated as a shocking twist in "Bad Wolf". However not only was it spoiled by the trailer for that episode but the Radio Times revealed this three months earlier when the series began.
The sneak peek for "Army of Ghosts" at the end of "Fear Her" all but gives away that the "army of ghosts" is made up of Cybermen. This was not actually much of a surprise when you consider that the fact had already been reported in various media. However, the Daleks also appear at the last minute of the episode, a fact that the show's creators went to great lengths to keep secret... only to be spoiled by a rather obvious shot of a Dalek Death Ray firing in the same trailer.
The best part of it is that the Dalek attack scene isn't even in Army of Ghosts - it's from the episode after that, Doomsday.
The American recut trailers on Sci-Fi are even worse about this. The trailer for Utopia gives away the last minute twist of the Master's return. Perhaps they thought Americans wouldn't be up enough on Doctor Who history to understand it when it came, so they spelled it out for us.
Oddly enough, they spoiled the same episode with the same line in the Australian trailers, a country that's been regularly airing Doctor Who since 1966.
This however doesn't excuse what they did for the fourth season finale: the very first thing they show is David Tennant's face, spoiling that he doesn't really regenerate, and it goes downhill from there. It's like they want Americans to pirate the show from the UK...
Subverted oh so very much in the BBC trailer for "Forest of the Dead."
Sci-Fi's version (as usual) pretty much ruins it, what with Donna being alive and River biting it.
The worst example in the history of the new Who is the episode Daleks in Manhattan. The twist ending of this is the revelation of the dalek-human hybrid- whose picture was on the front of the Radio Times.
Alternatively, for The Stolen Earth. The protagonists finally learn what has caused the Earth's movement, and that there is reason to be very very afraid, and the audience should also be very very afraid and surprised... except that the Daleks had been on the trailer for the entire series, broadcast a least once a night for the past three months, and hadn't yet appeared in a single episode.
However, it was played with quite well in Victory of The Daleks getting Radio Times cover. It got three, each showing a different Paradigm Dalek, but since these Daleks had three with the colors of the British Political Parties they played it off as for the election, one for each. Still spoiling, but covertly.
Oh, and the End of Time DVD menu is basically a montage of the first episode's cliffhanger, giving all the plot twists away. It's almost entirely made up of clips of the Master zapping around or taking control of the Immortality Gate or turning everyone into himself. Nice one, BBC.
The cliffhanger at the end of the opening two parter of Series 6 centers around Amy shooting a little girl in a space suit. The trailer for Part Two not only shows the girl alive and well, but also where the bullet harmlessly penetrated the space suit.
One of the trailers for The Angels Take Manhattan shows Amy and Rory falling. So the minute they get to the roof, you know what's coming....
The BBC website spoiled the return of the Master in the 2009 Christmas Special. In September.
The trailer for the 2008 series featured several foes, including, rather prominently, Daleks; come the end of the penultimate story, they still hadn't been seen, or even mentioned. Guess who's involved in the finale?
Drake and Josh: Commercials for the Finale, "Really Big Shrimp" were egregious users of this. One of them showed the entire resolution of the episode, which is everyone at Helen's wedding, Josh wearing the gold vestAND dancing with Mindy, (If that's not enough, another ad showed them kissing) and Drake performing his hit song, interwoven with clips form the episode.
Eastenders: Subverted Trope in the late 90s, when fake spoilers were inserted into trailers. One gave the impression that club owner Steve was going to be killed by his girlfriend, whereas the actual episode had it happen the other way around. Steve then framed his colleague, who subsequently escaped from prison and returned for revenge. The trailers for that episode implied that he had booby trapped several lightbulbs with explosives as revenge, but the episode had no exploding lightbulbs in it at all.
ER: One episode did this badly enough for TV Guide to call them out. The promos for one episode touted the return of Dr. Carter, even showing a brief scene. How long was Carter's actual appearance? Exactly as long as in the promos, using the exact same scene.
The Event: Ads for this show return following its Christmas break made a huge deal out of revealing the origin of the aliens on the show. The show itself does not reveal this until the very last 5 minutes of the last episode, about six episodes after the ads started mentioning it.
Falling Skies: For the episode "Grace" in the first season, the promo immediately before it showed the boy putting the harness back on his back. The boy doesn't actually put it on until 45 minutes through the hour-long episode.
Firefly: A well-known TV example would be the trailers for the pilot (if you can call it that, considering it was the last episode aired). The major act break at the half-way point of the two-hour episode was supposed to have been revealing what was in the box Simon was so anxious to keep secret. This was ruined by the fact that the FOX promos spoiled it from the get go, as well as showing the moment the box was opened in the opening credits.
Frasier: The producers had to pull a fast one on NBC in order to make this an Averted Trope. In the "Adventures in Paradise" two-part episode, Frasier finds himself at a Hawaiian resort in a room next to his ex-wife, Lilith. The second part ended with a dream sequence where Frasier was back at the resort, this time next to Shelly Long as Diane Chambers. The producers were worried that NBC would heavily promote the surprise cameo, so they shot the scene in secret and turned in a copy of the episode without the scene, only giving the real episode to the executives at the very last minute.
Gilmore Girls: During the last season, the teaser at the end of each episode showed the very last scene of the following episode. Technically, this may have been more misdirection than spoiler, though, as the final scene of each episode was usually unrelated to the main action of the story, and was itself a lead-in for the following episode. Which is to say, that after the final scene foreshadowed the next episode, the On the Next teaser that followed showed you what amounted to a teaser for the episode two weeks down the line.
Glee: Did this in a season 2 episode, showing Kurt, who has been at a rival school for ten episodes, standing at the top of a staircase yelling "Kurt Hummel's back at McKinley!"
Good Luck Charlie: Another Disney Channel mistake, before the credits for a new episode of this show started, a trailer for a Saturday block clearly said that Gabe won his class election, and showed scenes from the credits we had then yet to see.
Great Expectations: PBS' trailer for the newest adaptation gives away how Miss Havisham dies, even though the official website goes out of its way to stick "Spoiler Warning" on the production designer's discussion of that event.
Grimm is somewhat notorious for this to a point at which some fans stopped watching the trailers. One of the worst examples was showing that Cinderella/Lucinda was a Wesen even though the episode itself did its very best to draw the attention of the audience to another character.
Hannah Montana: The promos for the Forever episode "Hannah's Gonna Get This", spoiled the fact that Hannah ended up recording the song as a duet with Iyaz, something which doesn't happen until the last minute of the episode.
Heroes: Crossing over with New Media: watching NBC's online streaming of this show's episodes, instead of the broadcast, is only for spoilerphiles and people who click 'Play' faster than they read. Their single-sentence summary for "Angels and Monsters" manages to completely give away the ending of Claire's plotline: the guy kills himself.
Hollywood Showdown: When TV Guide Channel re-aired episodes (which was 30 minutes), they would pad out the show with commercials to make it an hour long. Towards the end of the hour, they would run the first five minutes or so of the next episode.
House: Previews occasionally seem to make this a Subverted Trope, by taking one of House's sarcastic lines from the next episode and implying that it is literal.
Local previews for the Australian broadcasting of House were often deliberately misleading, taking quotes out of context and hinting that the focus of the story was something totally different.
This subversion itself may have been lampshaded by a Season 4 episode where a documentary crew, failing to get House to utter anything serious, edits their documentary to make comments like "I became a doctor because of Patch Adams" look serious.
The preview for the finale of Season 6 on British TV spoils the surprise ending: Cuddy leaving Lucas and telling House that she loves him.
Polish DVD Box for 4th Season of the series is one of the greatest offenders of this trope. The whole point of the 4th season is "Who will be in House's new team". Some brilliant editor thought that it would be great if he placed the three new doctors (chosen in the middle of the season!) on the DVD cover.
Also shown on the back of the DVD cover in the British release (I've just had a look...)
While on the subject of the British Season 4 DVD boxset, each episode on the DVD itself is accompanied by a still from that episode. The still for 'Wilson's Heart' shows a distraught Wilson lying with Amber on her deathbed.
How I Met Your Mother: In the promo for the Season 4 finale, they showed a scene of Barney and Robin kissing, despite the fact that a major reoccuring plotline spanning the entire season involved Barney struggling with his feelings for Robin.
Done on a more minor level with most, if not all, promos for this show. Due to Wolverine Publicity, CBS seems to have gotten it into their heads that every single one of their promos should mainly consist of Barney doing something sleazy or eccentric, regardless of how prominently Barney actually figures into the episode's plot. Therefore, most of Barney's crazy stunts get spoiled in promos before the episode ever airs. On one hand, this makes Barney seem incredibly annoying in the promos and sucks the humor out of his scenes. On the other, it means the other characters' (usually more substantial) plotlines are ignored by the promos and therefore remain complete mysteries until the episode airs, sparing them from this trope.
iCarly: Commercials for the hour-long special "iPsycho", in which a crazy girl locks the trio in her basement, featured about no clips from the first half-hour of the episode. Instead, every commercial emphasized their friend Gibby coming to save them, which literally happens within the last 5 minutes.
Seriously, just about every episode does this now. Recently examples include spoiling the funniest (in an episode which wasn't especially funny to begin with), and 'climactic' scene in "iSpace Out".
"iGot A Hot Room" spoiled in the trailers that it was Carly's birthday, that Spencer burns down the room trying to do something nice for her, that Carly is upset at it, that Carly has a job as part of the episode, that Spencer rushes to re-do the room as a surprise with Freddie and Sam, the fact that Carly loves the new room, and what the new room itself looked like.
"iCan't Take It" aired nine minutes of sneak peeks for a 22 minute show, including spoiling the big secret about Sam's misdeed to Freddie, and the ending where Freddie saves their relationship and Freddie kisses Sam again.
In multi-parters they often have spoilers in the "Next time" segment. For example, the Next Time On for iStill Psycho part 1 spoils Nora's seemingly normal dad being in on the scheme and Mrs. Benson and T-Bo coming the gang's rescue.
Impractical Jokers: TruTV is getting really bad with this for this show. Often a commercial airing during the show will spoil who the loser for the episode is.
Law & Order: Sometimes gives away the twist in the commercial for it, or even in the preview right before they play the ep.
Law & Order: Los Angeles: Began running previews nearly a month in advance of its return from its winter hiatus stating that "Law & Order loses one of its own" followed by a montage of all the main characters, indicating that their first episode back would be a Tonight Someone Dies story with one of the main characters being the one killed while leaving viewers speculating about which character it would be. However, subsequent previews shown closer to the episode's airdate blatantly spoiled which character would be killed, even showing nearly the entire death scene.
Leverage: In the Season 2 episode that introduced Tara Cole, she was posing as their client's uptight lawyer to "audition" as a stand-in grifter for Sophie - a fact she didn't reveal to the team (or the audience) until the end. Unfortunately, that was given away in one of the promos that aired just prior to the actual reveal in the last segment.
Life on Mars (2006): Mere minutes after the last episode had finished on BBC One, fellow BBC channel BBC Three's 60 Seconds (presumably attempting to encourage viewers to watch the episode) announced that the final episode had been shown and, in under ten seconds, spoiled that Sam had been revived in the present and jumped to an apparent death to return to the '70s. It was followed by an (unscripted, one imagines) apology for those that had recorded the episode to watch later.
LOST: Happened in the Season 3 finale. The commercials for it showed Jin, Bernard, and Sayid tied up. In, the show, Ben ordered their deaths, and you hear 3 gunshots through the radio. However, the scene with them tied up did not appear yet in the episode, telling people preemptively that they were alive.
In the penultimate episode of the 5th season, Kate, Sawyer, and Juliet are seen leaving the island. However, the commercials for the finale show them back on the island. So much for that.
Creator/producers Damon Lindelof and Carlton Cuse have ordered ABC not to show any footage of season 6 in trailers for the next—and final—season. This is both because they want a large amount of suspense going into the show's conclusion, and because showing any footage at all would explain the results of season 5's massive cliffhanger.
ABC, however, did not listen to them and began showing new footage just a couple of days before the season's premiere. Due to how season 5 ended, almost any footage would have spoiled the basic premise of the season. They also spoiled specific things like the fate of Claire.
The previews for the last few episodes have done exactly this, and show absolutely nothing from the upcoming episode. It's nice.
"Everybody Loves Hugo"'s final scenes include Desmond being tossed down to a well, possibly to his death. Cue the next time trailer for "The Last Recruit", which shows Desmond alive and well.
To be fair, no one who watches LOST religiously would have had any doubt about his fate anyway. If we didn't see it, it didn't happen.
MadTV: Regularly did this with fake trailers (notably the one for the Rocket Revengers in Excito-Color movie). The narrator asks the audience various things like "Who will die?", followed by footage of that person dying, going so far as to even show the ending of the film. ("You'll have to pay big money to find out Tooka's secret; that she's carrying Tiny's baby!")
However, a year before MADtv debuted, local Seattle show Almost Live! did their own fake trailer, which you can see here.
Maury: This talk show does this religiously. In every commercial break before DNA test results, they try to build suspense by showing quick clips of the guests before and after the results. 99% of the time, they show the guests' reactions to the results, defeating the purpose of sticking around for the results. Sometimes, if the DNA test is for a more serious tone, like an adult daughter finding her long lost father, the clips fade into a commercial break without showing the reaction.
Considering that the crazies they get, their reactions would have come no matter what the result would have been.
Merlin: Series 4 of the BBC's show had a trailer for the first episode which showed Arthur carrying a lifeless Merlin. This is the cliffhanger at the end of the episode.
Interestingly, the series is also infamous for the unreliability of its trailers, particularly where interactions between Arthur and Merlin are concerned. There doesn't seem to be a middle ground.
On a similar note, the US previews note This US-based troper is unsure whether the UK preview was the same for season 4 opened with "The king is dead", successfully spoiling the third episode before even making headway on the season premier. Not to be deterred, it went on to display Merlin and the Dorocha that (successfully) attacked him and the aforementioned carrying scene. And also Uther's bodyfor good measure. Just to let us know they were serious.
My Babysitter's a Vampire: The promos for the season finale on Disney Channel pretty much spoil what was probably intended to be a Wham Ending, showing clips that make it blatantly obvious that Jesse bites Ethan and Sarah sucks the vampire venom out of Ethan, becoming a full vampire in the process.
NCIS: Los Angeles: Did this at the beginning of season three. After shooting Hetty at the end of the premiere, they went on to reveal that she survived the shooting in the promo for the next week's episode.
Although they do subvert this in the promo for the fifth episode of season four in which all of the footage is from the continuation of the episode airing two weeks later. Seriously, nothing that that promo happened in the episode. Not a damn thing.
NUMB3RS: The promo for the episode "Spree" made a big deal about Megan being kidnapped, as if the entire episode was about that. Almost none of the scenes in the promo are in that episode (they're in the next one, "Two Daughters"), and Megan isn't kidnapped until the last thirty seconds of the episode, as a cliffhanger. So... the trailer made the entire episode pointless, really.
The O.C.: Network Ten purposely ruined the shocking twist in the finale of the third season for Australian viewers. Instead of letting us think she was getting Put on a Bus ten decided to start showing ads three weeks before the finale saying " MARISSA... WILL... DIE".
Once Upon a Time: The show is terrible at this. Between the trailers and the lengthy promos ABC releases before every episode, you barely need to watch the episode at all. Most recently the trailers for season two episode 'The Doctor' gave away the big twist the writers had been keeping secret from fans since season one, that Doctor Whale's Fairytale Identity is Doctor Frankenstein.
One Tree Hill: The CW did this to themselves with a Season 7 episode. The On the Next preview that aired after 7x08 had Brooke's voiceover stating that she thought she could be pregnant. Later, the CW released the clip where she confesses this to Haley before a concert Haley will be playing that night at the town's nightclub. Then the CW releases a promo photo of Brooke drinking at Haley's concert, clearly revealing she wasn't pregnant.
Then there were the trailers for the season five finale which showed the cliffhanger-end in the preview.
Parenthood: The preview for an episode in Season 2 showed one of the main characters being involved in a potentially fatal car accident and implied that the episode's entire plot would be centered around the buildup to the accident scene. The episode itself had the character shown in the accident scene being involved in a completely unrelated storyline with the buildup to the accident only happening within the final 5 minutes and the accident itself being the very last shot before the credits rolled.
The trailer for series 3 of Primeval shows the scene where Helen shoots Nick.
Project Runway: Lifetime managed to kill the suspense surrounding the identities of this show's eighth season finalists by airing a promo for the next week's episode during the commercial break directly preceding that episode's elimination.
Oxygen Network is bad with this as well, in particular with their reality show Hair Battle. Gee, thanks for giving me a comprehensive list of everyone who is not going to be eliminated halfway through the episode, guys.
The VHS release Learning to Add and Subtract involves Big Bird needing to learn how to add and subtract "just in time", with Maria constantly asking "just in time for what" but Big Bird not listening and not answering until the very end. But those who read the description on the video box would already know that Big Bird had promised to teach Elmo how to add and subtract before he even knew how.
Single Father: Other David Tennant dramas can't escape this either. The Next Time trailers for this show repeatedly revealed major plot points. You'd think that for a show where a major, tension-creating plot element would be about who fathered whose children, you wouldn't give the answer away in the trailers. But no....
Six Feet Under: A DVD sleeve example - the UK boxset of the final series of this show not only shows a wedding photo of Nate and Brenda, but the entire cast at a funeral with Nate mysteriously missing....
Sliders: One episode had the title group land in a version of San Francisco where those in charge force everyone to use some kind of buddy system. If one guy does something illegal, the other one is killed. The area is also more prone to earthquakes than the normal San Francisco and everyone knows that a big one is imminent. Quinn asks one authority figure why nobody tries to leave because of this. The man tells him something the audience learned from the promos: this version of San Francisco is a maximum security prison.
Spartacus: Blood and Sand: In the trailer for the final season War of the Damned, there is a brief clip of what is clearly the final battle that spoils who does and doesn't make it to that point.
Stargate SG-1: Gets a Lampshade Hanging along with pretty much everything else on the episode 200. According to Wikipedia the twist they're talking about (Jack O'Neill's sudden appearance) actually made it to the commercials for the episode.
Vala: Wow. Nobody's gonna see that coming.
Daniel: No. There'll be spoilers.
Carter: Are you kidding? It's gonna be in the commercial.
Also occurred in the episode "Fragile Balance". A boy of approximately 15 years attempts to gain access into the SGC. After a little info checking, the boy convinces everyone that he is Jack O'Neill. A genetic test reveals that he's a clone which, for some reason, didn't age properly. Rather than let us find out about this little twist during the episode, the trailer showed adult Jack O'Neill stating it in no uncertain terms.
And more recently, a commercial for Stargate Atlantis promised you "won't believe what happens in the last five minutes..." before showing you exactly what happens. Of course, may also be a subversion as the commercial's description for the rest of the episode's plot is completely off.
The announcer notably said in that episode's trailer: "They fall into a surprise attack of the replicators!" as the trailer shows...a Wraith ship attacking.
Nothing beats a sneak peek into "The Lost Tribe" giving away who the new enemies are.
Another possibly attempted subversion was the commercials for the episode where Teyla poses as a Wraith queen, with scenes taken out of context to imply she would end up turning against the team. The possibility isn't even mentioned in the episode itself.
EVERY SINGLE promo for Stargate Continuum shows Ba'al being betrayed and killed by Vala/Qatesh, which is really supposed to be a surprise.
Star Trek: The Next Generation: The original trailers could be pretty awful about this too. Perhaps the most egregious was the trailer for the episode, "The Most Toys" which showed Data being captured by aliens, a woman alien offering to help Data escape, and a man incinearting the woman with a phaser. When the episode aired, the man was the villain of the episode while the woman was his loyal assistant...and was a major character throughout the entire episode, not doing her Heel-Face Turn and getting zapped by her boss until within the last 5 minutes of the episode.
Though it wasn't a secret that the search for Spock (pun intended) was on in "Unification Part 1", the fact that they found him was spoiled...because the last five seconds of the episode (with the big reveal, which would be followed almost immediately by "To Be Continued") was shown in the trailer.
Star Trek: Voyager: One immensely frustrating one occurred. At the very end (literally in the last five seconds) of the otherwise unrelated episode "Blood Fever," the crew discover a Borg corpse, setting up the next episode, "Unity," and the primary threat of the remainder of the series. It was pretty effective — it came completely out of left field and chillingly evoked one of the most terrifying enemies in the Trek mythos (regardless of how unforgivably Villain Decayed they would subsequently become). So what do the producers do? Why, they put that scene right in the trailer, of course.
Voyager's "The Chute" is a classic example. Paris and Kim are thrown in an alien prison, and about halfway through comes the revelation that they can't break out because the prison is in space. It's a very dramatic shot that would no doubt have been more effective if it hadn't been in the commercial.
Supernatural: The trailers are infamous for this. For example, the season 3's big cliffhanger ending of Dean being sent to hell was kind of ruined since the original episode promo showed Sam crying over Dean's dead body. More recently, the episode promo for season 7's "Repo Man" completely ruined the episode's big twist of the apparent victim actually being a villain and trying to let a demon that once possessed him once again inhabit his body by actually making it the focus of the trailer.
Super Sentai: Certain seasons of this show (such as Boukenger and Gekiranger) have a nasty habit of showing story spoilers in the "next episode preview" at the end of each episode.
In the Boukenger episode called "The Golden Sword," the Monster of the Week is Nigh Invulnerable and utterly tearing the Rangers a new one. The trailer reveals that in the following episode, a new character could turn out to be friend or foe and might even fall under the bad guy's control. Then it goes onto show the Rangers handily beating up the monster that was killing them in the current episode, then said new character joining in and later posing with the Rangers' Humongous Mecha. Not much is saved for the actual episode at all.
Thunderbirds: The episodes all began with a trailer showing what was coming up in the episode. It would generally show the perilous situation of the week, the series of events that led to said perilous situation, what cool machine Thunderbird 2 would unload and how the situation would be resolved. Which is essentially the whole plot. And it would be shown at the beginning of each episode.
In "Top Chef Season 7", on the second-to-last episode, Bravo stretched the episode. Instead of going from 10-11 pm, they had it run from 10-11:30 pm to increase the suspense of the final elimination. The viewer watches until 11 pm, and that is about where Judge's Table starts. Here's the slip-up: Bravo still has the ads going like the episode was 10-11. So on the commercial break where they are choosing between three contestants to be eliminated, there are the two winning contestants walking through a door on the preview for the finale. Guess who gets eliminated now.
"Top Chef Masters Season 2" was to select eight people to compete in the final round. This was the last selection round, and before the elimination they play a quick clip of the eight people in the final round. No point watching the ending anymore.
In almost all the seasons, Season 1 has ended with a "This season on Top Chef!" preview, where you might see Bob saying "I'm cooking at a baseball park!" So until that clip pops up, Bob is completely safe. And if Alice is saying "We have to cook at NASA." Then again, Alice has immunity til the NASA challenge, and the shock of this new challenge will be softened because you knew what it was anyway.
Top Shot: Subverted Trope for a few trailers, a reality show where competing marksmen are progressively eliminated. More than a few times the previews for the next episode, usually aired the commercial break before finding out who goes home, showed competitors that were eliminated.
For True Blood, it wasn't a trailer but the news that spoiled a Season 7 plot element. It was reported that actor Luke Grimes—who played Jessica's boyfriend James introduced in Season 6—was replaced because Grimes didn't want to act out a gay love affair with Lafayette. But the story broke shortly after the Season 7 premier, where the two were still platonic and only had a single conversation onscreen (about Tara's True Death and how James was brutally murdered before being made into a vampire) that didn't indicate any sexual attraction between the two.
True Jackson VP: A similar scenario arose in one special. The trailers posed the question of whether True and Jimmy will become a couple or not while almost simultaneously showing the two kissing, which happens mere seconds before the episode ends.
The Tudors: Even Showtime manages to do this. A trailer for season three aired before the season began showing Henry being introduced to his fourth wife, with a voice-over of how marriage to her would add military might to England, thus spoiling the mid-season plot point of Queen Jane dying. Granted, the series is based on real events, so this must happen eventually, it does put a rather rigid timeline on what should be a sudden and jarring event.
The last few seasons of this reality show has had several fight ending finishes shown during the commercial about the show just before it happens.
Also, they often tease a "special guest" showing up in the next episode and vainly attempt to edit around the actual person to keep it a surprise. This often fails (IE, Matt Hughes being clearly seen sitting on a bench in the background in one teaser).
Has occurred, sometimes inadvertently spoiling fights during the in-episode previews.
iCarly's sister show is very guilty of this too. The promo for "Cat's New Boyfriend" gave away that Cat was dating Tori's ex, Daniel, that Tori sprayed cheese on Cat and Daniel, that Tori kissed Daniel, and that Cat punched Tori in the face.
While not as bad a letout, the first promo for the episode "Robarazzi" shows Robbie being worried about his blog, then humiliating Tori after the video of her playing with her pimple is let out. Then, in another ad promoting the block it was going to be aired in (with iCarly and Big Time Rush), it shows Tori and Jade filming Robbie in his gym towel against his plea, implying that they set up a revenge plot, which happens about three minutes from the end of the episode.
The Walking Dead: The Season 2 Comic-Con trailer shows a quick clip of Guillermo, the leader (and caretaker) of the retirement home in the first-season episode "Vatos" lying dead on the ground, while Rick and the others fend off a contingent of walkers massing around the area near his body.
Previews for the episode "Clear" averted anything that would spoil The Reveal, which harked back to an episode from some time ago, and which formed the basis of the entire episode. Too bad the Previously On blew that because whoever is in charge of it thought it was worth protecting new viewers from Continuity Lockout with a brief, out of context flashback at the cost of ruining the surprise for anyone else.
Warehouse 13: Averted Trope, not showing Artie in ads for season 2, since he is supposedly dead. They did the same for Myka in season 3 and Jinks in season 4.
Wheel of Fortune: In recent years, this show of all shows is guilty of this. The show uploads a preview of next week's shows on Sony's website every weekend. Nearly every preview shows contestants landing on or picking up prizes, the $10,000 side of the Mystery Wedge, or the Million Dollar Wedge. Occasionally, similar previews air on TV.
On October 13, 2010, one preview that aired near the end of the show was devoted entirely to a woman picking up the Million Dollar Wedge, complete with suspenseful music and an announcer hinting viewers that she would win the grand prize. When the episode in particular aired, she lost the wedge to a Bankrupt.
During May 27-30, 2013, the show's on-air promos and online trailers both were pretty heavy with spoilers towards a contestant winning $1,000,000.
The promo for the "Wizards vs. Werewolves" special showed Alex's new boyfriend, Mason, (who was introduced in the previous episode) screaming in front of a full moon as if he was about to undergo some kind of a transformation into a werewolf. Guess what?
The promo for the episode "Moving On" showed that Justin would miss Juliet, Alex would come up with some plan, and even had "Juliet" saying "I'm not Juliet." What happened here?
Also the episode "Everything's Rosie For Justin". Not only was it advertised as the first episode in the "Wizards vs. Angels" trilogy, but the promos featured her with angel wings and Justin saying "She's an angel". Them finding out about Rosie being an angel actually happens near the very end of the episode, and is probably supposed to be a surprising twist.
The title "Future Harper" - three guesses as to who H.J. Darling is, and the first two don't count.
The World Series Of Pop Culture: VH-1 spoiled a match this way: One of the semifinal matches spilled over into the final episode...and the preview trailer for the finale spoiled who wins that semifinal match. Oops...
WWE are generally fairly good at averting this, since RAW airs live & SmackDown is on a three day tape delay. However, there have been times when WWE.com has spoilt what was going to happen on SmackDown days in advance, but this is namely when a shock title world change has occured on the show.
They did, however, spoil the "Mr McMahon's Bastard Offspring" plot by airing a promo teasing The Reveal that ended on a shot which rapidly flipped through various members of the WWE roster, only to end on Hornswoggle & keep the image on screen for a few seconds. When RAW aired, guess who was the bastard.
In an aversion, the descriptions of some of the last few episodes of Breaking Bad are extremely vague, such as one that says "Things shake up for Walter in unexpected ways". That's as specific as they could get without spoiling something.