Second-season 24 episodes featuring the returns of Sherry Palmer and Nina Myers featured the actors' names in the opening credits. By the end of the season, they began leaving out returning actors' credits until the reveal.
They did, however, spoil a surprise again in the opening credits of the season 5 premiere. At the end of season 4, Tony Almeida and Michelle Dessler left CTU together to settle down and start a new life, yet in the first episode of the new season, Carlos Bernard is listed with the regulars and Reiko Aylesworth is listed as a guest star. By the end of the episode, Michelle is dead.
In season two, Sherry Palmer left during the 14th episode and was missing from the series for several episodes. She returned in episode 21 in a surprise cliffhanger at the end of the episode. However, her actress was listed in the opening credits, so you knew that Sherry was going to be showing up sometime in the episode.
"Live Another Day" may contain the most egregious example yet: In the 10th hour, Cheng Zhi's actor Tzi Ma is credited. Guess who turns out to be the Big Bad of the whole season in a surprise twist?
In Season 1 of The Amazing Race, the opening titles showed actual locations teams would be going to and tasks they would be doing, making it quite easy to work out whether certain teams were going to make it through to the next leg of the race. Seasons 14 and 15 are the only others to show shots from the season during the credits, and they were only ever shots from the current or previous episodes.
The opening credits of the first series of Angel include a brief clip of Doyle's death. But it's not really obvious that's happening until you've already seen the episode.
Angel also subverts this in the episode after Doyle's death by keeping the credits the same. Wesley would not be added to the credits until the following episode.
Angel averts this a little later with the appearance of Julie Benz as Darla in the Season 1 finale. The actress was not credited at the beginning; instead, she's listed as a "Special Guest Star" at the end of the episode. Listing her at the beginning would have tipped off everyone, and it was a great surprise.
And then in the season 5 opener, the opening credits not only conspicuously lack Cordelia and Connor, but proudly boast James Marsters as Spike, including him in virtually every clip of the opening, even though at that moment he's still officially "dead" from the Buffy finale, and he doesn't show up until the last minute of the season premiere.
Observant viewers of The Apprentice UK have been able to deduce who is probably going home/safe from the montages at the beginning, as they preview some of the later episodes.
The third season showed Starfuries fighting Starfuries, which signified a major change in alliances (and wouldn't happen until half way through the season).
Another B5 example, this an aversion that didn't stay averted: at the end of the second episode of season 2, a character who hadn't been seen yet that season was revealed to have a whole new look. In the original run, the first two episodes' credit sequences showed her as she had looked previously, and all later episodes showed her as she now looked. Unfortunately, in subsequent runs and the DVDs, the later credit sequence was used for the first two episodes, blowing the surprise completely.
Yet another spoiling intro is in the fourth season: G'Kar saying that it was the year "we took back what was ours." Maybe it's tradition.
Also, when Anna Sheridan turned out to be Not Quite Dead, the guest star credits at the beginning listed "Melissa Gilbert as Anna". Leaving off the surname might have worked, as Anna Sheridan had previously been played by a different actress, if Gilbert weren't Bruce Boxleitner's real-life wife.
A more obvious example of this trope is the guest star credit "Julie Caitlin Brown as Na'Toth", when she turned out to be Not Quite Dead as well. Unfortunately, they couldn't pull the same trick again as Na'Toth had Only One Name.
Battlestar Galactica has a "previously on" bit before each show, which allows you to infer exactly what situations will be present during the episode due to what they choose to show you during the opening scrawl. The scenes shown could be from many, many episodes ago, relating to events that have not been referenced since, making it obvious that the story arc would be continued in the following episode.
Each episode's title sequence ends with a montage of scenes from the current episode, wilfully courting this trope. Word of God is that the montages were introduced in homage to Space: 1999, which did the same.
BSG has a weird habit of showing clips in the 'Previously on' that never actually happened, either things that were edited out of previous episodes or scenes featuring guest stars where this was their first episode.
Except not really once the miniseries and webisodes are taken into account. Anyone who started with Season 1 would be very confused about half the interactions, unless they had seen the miniseries-that-really-isn't-anymore
BSG specifically avoids this in the Season 3 finale "Crossroads, Part II" - Katee Sackhoff does not appear in the opening credits, but is credited at the end of the episode, after Starbuck returns from the dead.
They do the same thing in the season 4.5 premiere with Kate Vernon as Ellen Tigh.
Beastmaster's opening credits specifically refer to the main character, Dar, as Last of His Kind, despite the fact that his girlfriend was from the same tribe. He doesn't become last of his tribe until the first season finale, when said girlfriend gets Killed Off for Real.
The credits for the second-to-last episode of Breaking Bad, "Granite State", reveal the return of Gretchen and Elliot, the latter of whom had not been seen since season one.
Similarly, "Felina"'s credits inform viewers that Badger and Skinny Pete will be appearing one last time.
Buffy was very egregious with this. Angel's actor David Boreanaz was listed in the first episode of season three, despite being sent to a hell dimension at the end of the previous season. Actors whose appearance on the show should be a twist (Oz when he had left, Buffy's mom after she died, and so on) would almost always be mentioned up front.
The opening credits of the two-hour Buffy premiere didn't include Xander's friend Jesse, so it pretty obvious that he wasn't going to survive. Joss Whedon had wanted to include him but they couldn't afford to make two different opening titles so this was nixed but he always claimed he wanted to do it. As a consequence, when Tara was added to the credits near the end of Season 6, that made her departure an easy guess for those who knew about the Jesse thing.
Angel averts this with its season two finale. It guest stars Alyson Hannigan as Willow, who shows up at the Angel Investigations office to inform Angel that Buffy has died. Since she only gets one line of dialogue, she was relegated to end-credits status. The same trick was used for Anthony Stewart Head as Giles in the Buffy season 6 two-part finale.
The 3rd season opening of Buffy shows Faith before she even makes her first appearance. Averted in season 1, it doesn't hint that Angel is a vampire.
The opening credits for the first episode of Angel Season 5 feature James Marsters, so you're kind of not surprised when Spike "unexpectedly" comes back from the dead at the end of the episode.
S2E2's opening mentions guest star Matt Bomer, who shows up at the end of the episode as a massive surprise! Unless you paid attention to the opening credits.
Even more egregiously, and once again involving Matthew Bomer, was his being listed as a guest star in the opening of the first season episode "Chuck Versus the Imported Hard Salami". At a time when Bryce Larkin is still believed to be dead, spoiling The Reveal that the container Fulcrum was attempting to smuggle in contained the very much alive Larkin.
Fridge Brilliance: At the time, Bryce had already appeared in another episode after his "death", in the pilot albeit only in a flashback. After being credited in this episode as well viewers would likely assume that it would merely be another flashback with him, instead leading to the reveal where he turns out to actually be alive.
Community has a spoiler ending in the penultimate episode of the second season, when the Dean of a rival school is listed in the credits. He is disguised as an ice cream mascot and masterminding the entire situation.
Criminal Minds did this big time in Season 7. During the previous season, Paget Brewster's character was written off by apparently killing her and eventually revealing that she was in Europe under witness protection, heavily implying that her character was never coming back. Well, guess whose name and face were prominently featured in the opening of the first episode in Season 7?
Each episode of Cupcake Wars has a customized opening explaining the day's theme, including a shot of the two finished cupcake displays.
There was a slight Spoiler Opening for the third Doctor'sfirst episode, where they went to great lengths to keep Jon Pertwee's face hidden through most of the episode to make the Doctor's new appearance a surprise... despite the fact that Jon Pertwee's face just so happened to be the very first thing shown in the opening credits of the same episode.
The classic series tended to have issues with spoiler closings in serials featuring the Master in disguise if they credited Anthony Ainley, people would immediately know which character was secretly the Master. As a result, they took to using bizarre pseudonyms like "Neil Toynay", which apparently caused some trouble and confusion elsewhere in the BBC.
The same thing happened for the Fourth Doctor in "Robot". He regenerates in the first minute of the episode, but nevertheless the first shot we see of his face is in the opening titles, wearing the outfit that he has a Costume-Test Montage and funny Showing Off the New Body gag before we should get to see it.
In a slight variation, a scene for Matthew Waterhouse was deliberately written into the serial immediately following "Earthshock" so that they'd be able to credit him in listings magazines released before "Earthshock" was broadcast, masking Adric's death.
One new series episode had a carefully-obscured monster who never speaks in a recognisable voice, and all the clips featuring it are shot from its point of view to obscure its appearance until the time the Doctor first sees it. It's referred to by the ambiguous name "Metaltron" and there's even a careful Red Herring about its nature early on when the Doctor spots a Cyberman head in a glass cabinet (Cybermen being made out of metal). Unfortunately, the title of the episode, shown in massive text at the opening, is "Dalek", which leads to the viewer sitting around waiting for the Dalek to show up.
The classic series was prone to something similar, as it sometimes ruined the cliffhanger at the end of part one. The shock revelations of the featured villains in, for example, "The Evil of the Daleks" and "The Sontaran Experiment" weren't exactly shocks.
Though, to be fair, many of the story titles in the early years were added later, although that had changed by the time of Evil. The Sontarans had debuted the previous season, although as the character was treated as a single villain many viewers had forgotten what his race was called. The most oft-derided example is "Planet of the Daleks", with the part one cliffhanger being that the planet has Daleks on it.
The opening of the first episode of Farscape avoids doing this by removing the Opening Narration used for every other episode, which recaps the first episode's plot.
The revised opening for Season 3 did spoil that Aeryn wasn't dead by featuring her prominently in the opening sequence despite theoretically dying the previous season. However, Stark didn't start to appear in the opening until after he had returned.
In the first episode of Firefly aired, the crew (consisting of everyone in the credits) has already been assembled. However, the unaired pilot shows how three of the crew came to join. On the DVD order, the pilot comes first, but uses the credits created for the season as aired. This renders a huge plot point of the episode - finding out which of three passengers is an enemy spy - completely redundant, as two of them are listed in the opening credits. The credits also clearly show River in a Box, which is intended to be a major twist.
The Fosters subverts this in the season 3 opener. Season 2's cliffhanger saw a car running a red light and T-boning the cat that Ana, Jesus, and Mariana were in; the final scene has Steph hear about the accident over the scanner and report "one dead". Jake T. Austin is missing from the credits, so it's easy to assume that Jesus died. Season 3 then opens with Steph waking up in a panic. While the rest of the episode makes it clear that the accident really did happen, if there was a fatality, it could only have been the driver at fault; all named characters survived. Jesus merely left for boarding school, which was fully set up in the previous season, though the Season 2 finale was Jake T. Austin's last episode.
The show opens every episode with an Emmy-winning, CGI-animated sequence showing a map of Westeros, zooming in on certain cities and towns. The cities shown are major settings for the episode to come and change periodically as new locations are introduced in the story arc. Often, however, the sudden appearance of an unfamiliar location in the opening credits - or the disappearance of a familiar one - has served to telegraph major changes in setting occurring in the episode.
Only actors who appear in the episode are credited during the opening, indicating which characters will be not be featured. This has also on occasion spoiled the return of unexpected characters, such as the first two episodes of Series 6 crediting Kit Harrington, he does however appear as a corpse, something the show has done before, masking his return a bit.
In season 6 episode 7 "The Broken Man", there's actually a teaser before the opening credits, rare for the show, which reveals that Sandor Clegane (better known as The Hound) is still alive in the Riverlands. This was ostensibly so that The Hound's survival wouldn't be spoiled by Rory McCann's presence in the credits.
The show typically opens with a "previously on" segment cutting together various clips to remind the viewer of all past events relevant to the stories in the episode, the result being an effective spoiling of what stories are going to be included in the episode and hinting of potential plot twists. I.e. if the opening shows Yara telling Baelon that she's going after Theon, don't be surprised if the episode includes Yara arriving at the Dreadfort to rescue him. Or if the opening includes Ned and Robert discussing Rhaegar Targaryen's supposed kidnapping of Lyanna, don't be surprised when the episode includes the conclusion of Bran's Tower of Joy vision and gives The Reveal of the popular fan theory that Lyanna is actually Jon Snow's mother.
Heroes: The episode that Rebel's identity is revealed, cast member Noah Gray-Cabey is listed in the opening credits. Long-time fans would recognize that he played Micah Sanders and would then feel free to have a good guess at who Rebel is judging by the ways he has been helping and communicating with the other characters even before he actually appears in the episode.
Averted (sort of) in Home and Away where Noah is still credited despite the fact Sarah Lewis shot him dead in the previous episode. This was to hide the identity of who had been shot and only reveal it in the episode. That and Noah does appear in that episode (though it's his ghost and only Hayley can see/speak to him) and the next one. It isn't until after then the credits are altered.
At the end of Season 3 of House, all three of his fellows have either been fired or quit. It's rather hard to believe that they are permanently gone, however, when the opening credits for the first episode of Season 4 not only still lists all three actors, but also still ends with an iconic scene of the fellows walking with House. However, the show then turns this trope on its head by continuing to keep the exact same opening for the next two seasons, without adding the new fellows to the opening credits (even after they are permanently hired and are in roles that would merit opening credits in most shows), and despite Chase and Cameron having fairly small roles all through Seasons 4 and 5.
A notable aversion occurs in the season 4 episode when House tries to guess who Wilson's new girlfriend is. In the closing moments of the episode, House secretly follows Wilson to his date and refuses to move until she shows. Surprise surprise, it's a very unlikely recurring character from the beginning of the season (Cutthroat Bitch, played by Anne Dudek). In all other episodes Dudek's name is listed after the opening credits, but that would've been a spoiler in this case so she's listed first in the closing credits.
Amber's later appearances as a hallucination (season 5) weren't spoiled; Dudek was only credited at the beginning of the episode in the middle of that story arc, not at the beginning nor for her surprise re-appearance at the very end.
Averted in Season 5, where Kutner's hallucinatory return would have been spoiled if they'd "bothered" removing Kal Penn's name from the credits when he left early in the season and Kutner died.
Kamen Rider Ryuki averts this trope by masking over the eyes of a number of people would would be suspected to be Kamen Riders, if not for the fact that they don't even appear in the show.
It's American counterpart, however, plays it half way. While it spoils all the riders (save for Eubulon/Alternate), it does not spoil the men and woman behind the masks.
Recent Kamen Riders love spoiling the movie(if it's currently airing in theatres) in their openings
Kamen Rider Kuuga was especially bad at this. Pretty much everything appeared in the title sequence 4-5 episodes before its debut in the show.
Kamen Rider Fourze showed all the members of the Kamen Rider Club in the opening since the first episode. Since it's on this trope's page, you can guess that they weren't part of the club when the first episode started. In fact, it takes 10 episodes for all the members to join. (12 if you don't count Kengo joining until he becomes Gentaro's friend).
Kamen Rider Gaim did it too. The riders and Arms had a Debut Queue in the show, and things spoiled in the title sequence include Baron, Ryugen, Zangetsu and Suika Arms.
Kamen Rider Build as well, perhaps to an even greater degree than the other examples. The opening of Build shows countless advanced scientific breaking through and destroying the Sky Walls as Build looks on. [[spoiler: The finale features the very same thing occurring, except with The Reveal that it was Build himself who was causing that very event in to create a new world without the Sky Wallsever happening in the first place by fusing the current world and an parallel one as Build completely destroys Evolt.
An episode of Law & Order reveals Ned Beatty as a guest star. When he shows up as the trial judge for the case of the week, you already know that his character is going to have significance to the plot beyond sitting on the bench and ruling on a few objections.
The Legend Of William Tell The opening credits spoil Drogo's appearance - he doesn't show up until the second episode, and even then he doesn't join them until the very end. Also, Seth, one of the group who breaks out of the mine with Will, isn't in the credits at all, so no one's surprised when he dies saving Aruna. In a more minor example, it also shows Vara laughing and happy with the team, proving that she eventually comes to like them.
Lost has done this several times: someone gets shot and is left for dead, yet remains in the credits. Oh, I wonder if they'll survive!
For example, in Season 6 Desmond's actor, Henry Ian Cusick is in the credits all season, long before his eventual "surprise" appearance. Although considering the fact that the creators pretty much told everyone before the season started that the actor would be returning, this may have been their way of keeping the surprise of WHEN they were returning rather than THAT they were returning.
Though it should be noted that Lost subverted this trope more often than playing it straight. An actor still appearing in the credits of this non-linear show doesn't necessarily mean they're still alive:
Boone is the first main character to die, yet Ian Somerhalder remains in the credits for three more episodes until the end of the season, appearing as Boone's corpse and in flashbacks.
After Shannon is shot, Maggie Grace remains in the credits for two more episodes, appearing as her corpse.
When Ana Lucia and Libby get shot, both Michelle Rodriguez and Cynthia Watros remain in credits for three more episodes until the end of the season, appearing as corpses, hallucinations and in flashbacks.
After Eko is killed, Adewale Akinnuoye-Agbaje remains in credits for one more episode as his corpse.
Notably, after Charlie's very prominent and very emotional death Dominic Monaghan stays in credits all the way through the premiere of the next season. He is still dead and only appears as a ghost though.
Similarly, while Juliet appear in absolutely no position to survive the end of Season 5 (she manually detonates a freaking hydrogen bomb), Elizabeth Mitchell still appears in the credits of the Season 6 premiere, though as a guest star instead of a regular. Turns out Juliet's wounds were lethal after all and she finally passes away after saying the Final Words.
Following Daniel Faraday's death Jeremy Davies remains in the credits for the remainder of the season, again as his character's corpse.
Thanks to non-linear story-telling, viewers saw John Locke's corpse way before the character had the chance to actually meet his demise. Either way, Terry O'Quinn remained in the main cast and was seemingly resurrected... only to slam the viewers with a twist and reveal that he was dead all along, his likeness used by the shape-shifting Man in Black. For almost two season straight, O'Quinn primarily portrayed the MIB, only showing up as Locke in Flashbacks, Flash Sideways and as a corpse.
Finally, in Season 6 Naveen Andrews, Daniel Dae Kim, Yunjin Kim and Zuleikha Robinson all remain in the credits until the end of the series even after their characters are killed, keeping them for the "parallel world" Flash Sideways storyline.
Another British example from Misfits - the opening episode features six teenagers starting their community service on the same day. All of the promotion materials and the opening credits feature five of those teenagers. Guess who gets hacked to pieces with an axe 20 minutes into the first episode! Also potentially with Nathan's 'superpower' - the first few episodes haven't shown his power, and his curiousity over what ability he'll get is a running subplot. Except that the opening credits show animations of everyone's power, including his.
With regard to Nathan's power, the image in the opening credits actually constitutes an aversion of this trope. The title sequence depicts what looks like a wolf shadowing him, which indicates some form of lycanthropy as his power. However, it turns out that Nathan is not a werewolf; he is in fact immortal. The wolf-like creature in the opening sequence is likely a reference to his step-father's condition, or perhaps a symbol of death (a black dog, possibly) and the fact that its claw misses him is indicative of his immortality. Admittedly the image is somewhat ambiguous, but it was probably intended to confuse the audience rather than provide them with blatant spoilers.
Mission: Impossible. The old TV series' opening credits has a montage of basically every major plot point for the episode. Of course, the viewer has no idea what any of it means at that point.
The Season 8 opening to Mystery Science Theater 3000 spoiled the return of Pearl Forrester (at the end of the first episode) and the "chase across the galaxy" storyline (several episodes in). This is probably because season 8 was really the only season to have a plot, at the insistence of Sci-Fi Channel execs. It should also be noted that, when the episode originally aired, it didn't have that opening. It showed Mike and certain Bots returning from the edge of the universe and that was it.
The title sequence for MythQuest contains expository information about Gorgos, the antagonist, that wouldn't be revealed until the fifth episode. It makes watching those first few episodes somewhat painful.
NCIS spoils the introduction of two of their main characters: The opening credits for the very first episode features Sasha Alexander, spoiling the surprise of her joining at the end of the episode (it could count as a Late Arrival Spoiler, but it wasn't when the episode first aired), while the opening credits for the fourth episode of season three show Cote de Pablo, before her character is reintroduced and made a part of the team.
The opening credits in the third season of Once Upon a Time both plays straight and subvert this with two characters. At the middle of the season Rumplestilskin dies in a heroic sacrifice but the actor's name still appears in the credits of later episodes. Thus his come back wasn't a big surprise. Then Neal/Baefire dies, the actor's name still appears in the credits... but the character only appears later in two flashbacks.
Season one of The Outer Limits frequently had previews of the episode before the opening sequence. These previews often gave away important plot points or the appearance of monsters that the actual episode slowly built up revealing.
In the Season 1 opening of Oz is a shot of someone wearing the mask and restraints of a prisoner about to be executed. Though his face is hidden, we can clearly see a "MOM" tattoo on his hand.
The credits sequence of Penny Dreadful gives a huge hint about Ethan Chandler: a juxtaposition of him and a wolf's fangs. Many viewers correctly deduced that he was a werewolf before the reveal in the first season finale.
The season 2 opening of Prison Break leaves out three of the fugitives known as the Fox River Eight, one of which had been a regular in the first season, all of whom are dead before the end of the season. It also leaves in prison guard Brad Bellick, somewhat foreshadowing the fact that he sticks around as a bounty hunter after he gets fired from the prison. On the other hand, Robin Tunney is credited to hide her character's death at the end of the first season.
Red Dwarf, debatably. Series VII's opening credits feature alternate Kochanski. However, as she was being played by a different actress, she wasn't immediately recognisable.
The Opening Narration of season 6 of Robot Wars sometimes ended up with this as it was played over clips from the upcoming episode and essentially gavea away who'd end up fighting who after round one (probably the worst example of a spoiler in this sequence was asking the question "Will Thermador end up in the pit?" over footage of Thermador driving into the pit!).
During the Schloss Einstein title song, short video clips of the major kid characters are played. (The title sequence is the same for all the episodes of a given series.) Sometimes, who is in the title sequence spoils the resolution of a plot line. For instance:
At the beginning of Season 14, we know that Ronja will return to Schloss Einstein because she appears in the title sequence.
Max originally isn't supposed to go to Schloss Einstein in Season 11 he's just there with the rest of his family, who are dropping off his two sisters there before leaving to spend a year in China. He soon convinces his parents to let him stay and become a student there.
The opening monologue in the latter episodes of Series 6 of Shameless (UK) shows the dramatic season-ending Mandy-killing explosion, which somewhat undercuts the suspense of the final episode when Mandy is desperately trying to stop Maureen from lighting a match to blow up her house.
Spooks averted the trope by having Rupert Penry-Jones appear in all the publicity for Season 7, including doing extensive interviews, appearing in first place in the titles, in group shots etc. His character was killed off in Episode One, and it was a HELL of a shock.
And yet despite displaying awareness of this, the very next season released a publicity photo of all of the characters at their desks. Or rather Ruth, Tariq but no Jo or Malcolm.
The opening used in the pilot, "Children of the Gods", was recycled from the movie and didn't show any scenes from the series itself. Which would be a good idea if they also left Christopher Judge's name out to avoid spoiling Teal'c's defection. That is, for those who recognize Christopher Judge as Teal'c.
Also a major spoiler in the pilot is the fact that Kawalsky, a member of the SG-1 team who was also a main character in the film, isn't credited as a regular in the series. He winds up possessed by the end of it and the main drama of the following episode is whether the team can save him, but given he's only a guest star it was pretty clear that they would fail. Sure enough, by the end of the second episode he's been killed off.
In one episode while Daniel was dead and Ascended, Michael Shanks appeared in the beginning credits as a guest star. However, it turned out that he was playing his other character, the Asgard Thor. Another episode had Thor make a surprise appearance in the end, for this scene, Shanks was put in the ending credits.
Similarly, after Daniel Descended at the end of that season, the opening credits for the following season included Michael Shanks as Daniel Jackson, but not Corin Nemec as Jonas Quinn. It was not supposed to be revealed until the end of the second or third episode of the season that Jonas would leave SG-1, and events also threw into questions whether or not Daniel would actually stay. Not that the audience really questioned that. Status Quo Is God.
Like the above example, the first episode of season nine includes Mitchell, Daniel, Sam, Teal'c and Landry, even though it was (supposed to be) uncertain whether or not Daniel, Sam, and Teal'c would actually come back to SG-1, while Vala was being played as a returning character. The latter was a "special guest star" until about a quarter of the way through the season, and also made opening credits the next year. However, Sam didn't return until several episodes into the ninth season.
Stargate Atlantis did not have a title sequence for its pilot, which would have revealed the plot. Season Two averted this trope by not having a title sequence at all for the first several episodes in, by which time we had discovered who was MIA and who was joining the cast. Too bad they forgot this for the state of Season Five.
Also, there's an easy-to-miss one early in the fourth season of Atlantis. The show reverts to using the previous season's opening sequence (or the credits in it, at least), including Dr. Carson Beckett, who died around the end of the third season. His name is only in the opening credits for one episode, though, so you can chalk it up to a mistake... unless you've previously read about the "Oh, Michael's got a clone to replace the dead original" thing that's going to happen.
In the fourth season, however, there's one that's very difficult to miss. The third season ended with Elizabeth Weir being injured. In the fourth season opener, Torri Higginson, who plays Weir, was left out of the credits, as was Paul MacGillion as Beckett, who had died partway through the third season but been left in until the end. Higginson was replaced by Amanda Tapping as Sam Carter.
The series carefully avoided this several times. The first episode opening is slightly modified, so as to avoid showing you the wormhole (which won't be introduced until half way through the episode). The third season uses the second season opening, with the Defiant, which is introduced in that season, not shown in the opening until the fourth season.
Unfortunately they screwed it up in the seventh season premiere:
The same happens in the fourth season opener, with Michael Dorn listed as Worf in the opening credits, giving away not only the fact that he makes a "surprise" appearance 20 minutes later but that he joins the station's crew 90 minutes later.
In the episode "Heart of Stone", the Female Changeling posed as Major Kira in an attempt to persuade Odo back to the Great Link. In order not to spoil the ending, the actress agreed to be listed in the ending credits, instead of the opening guest star credits.
Nicely averted by the episode "Duet", in which the actor playing Marritza is listed, as Marritza, immediately after Odo identifies him by name. Actually a Double Subversion: for most of the episode, we are led to believe that the character's real identity is Gul Darhe'el, and if this were true, the credits would be deliberately misleading; however, it's revealed that his real real identity is Marritza after all.
Star Trek: Voyager did this in the fourth season, with its second episode being advertised as one where "a crew member leaves forever, and you'll never guess which one." Except that the opening credits for the first episode no longer showed "Jennifer Lien as Kes", replacing it with "Jeri Ryan as Seven of Nine".
Voyager also did this in the second season premiere, involving an episode where the dramatic tension and "big reveal" of the episode revolved around the crew of the eponymous ship discovering Amelia Earhart and other humans abducted from the 1930s on an alien planet. Too bad the big reveal was really revealed in the opening credits by listing the critical "surprise" role.
In the very first episode, the fact that the first officer and helmsman aren't in the opening credits (and neither is the chief medical officer, although anyone who doesn't recognise Robert Picardo might think he's "The Doctor"), but the crew of another ship, the convict acting as a guide to Voyager and the two aliens they meet are. It's pretty clear who isn't going to stick around and who is going to end up joining Voyager's crew.
Stranger Things: Anyone wondering if Eleven really died in the Season 1 finale has their answer when Millie Bobby Brown's name appears in the opening credits of S2. Said character appears at the very end of the first episode.
In the "Then" part of Supernatural, before the episode, when they recap the events up until that point, they usually focus on important plot points to that specific episode.
Super Sentai and Power Rangers sometimes have a habit of putting the Sixth Ranger in the opening credits before he is recruited. This doesn't make much of a difference, though, since the new Rangers' identities are usually pretty damn obvious anyway (due to colours and other reasons).
The first episode of Power Rangers in Space averted this by hiding the identity of the new villain of the season, Astronema, by putting the previous season's villain, Divatox, in her spot in the credits. Divatox did appear in this episode, but only made cameos in subsequent ones. Andros was also absent from the first episode opening credits.
Mighty Morphin' Power Rangers avoided Spoiler Openings by introducing them later, for example series 3 opening from part THREE of Ninja Quest, Alien Rangers from part TWO and things like that.
Power Rangers RPM both averted it and played it straight. While the first opening doesn't quite spoil the facts that Ziggy will become a Ranger and Dr. K is a teenage girl (a very brilliant one, yes, but still a teenage girl), it does spoil Dillon becoming Ranger Black (then again it didn't happen until episode 2). Then, later, it spoils Gem and Gemma being the Gold and Silver Rangers.
In an odd case, when the opening of Power Rangers Lost Galaxy was Mike, it also showed scenes of the Centaurus and Stratoforce Megazords (not seen for another 3 episodes), the Red Capsular Cycle (not seen for another ten episodes), and, most egregiously, Leo's Battlizer, which would not make its debut for approximately twenty episodes.
A minor example: Samurai Sentai Shinkenger: the first episode had the theme song but not the opening credits. Unless you were a (most likely adult) fan who was following the news about it (or the very likely scenario, for Japanese children, watching Go-onger's last few episodes), this preserved the surprise of who would become the non-Red Shinkenger (The theme song was playing after Shinken Red had already henshined).
When the opening for Zyuden Sentai Kyoryuger was updated, it contained everything missing from the pre-movie version...including Daigo's Super Mode, which debuted in the very episode the new opening premiered. Also, the new villain, Endolf, wasn't in the opening. Come episode 29, he gets Put on a Bus.
Series premiere "Everything Changes" averted this fantastically. All the publicity material showed Indira Varma's character to be part of the gang, and she is quite a well-known actor, so everybody was very surprised when she died at the end of the first episode.
Averted for new viewers with "Adam", where a alien who implanted himself into the cast's memories is included in the opening along with the rest of the cast.
Averted in the second season of Veronica Mars, where Duncan's brief cameo in the season finale, where it's revealed that he ordered the execution of Aaron Echolls, was a surprise largely because he hadn't been removed from the opening credits after he was written out. As if to make up for this, in the third season finale the reveal that Jake Kane is the head of The Castle (Neptune's secret society) is undercut by having Kyle Secor listed as a Special Guest Star at the start of the episode.
The Season 3 opening Warehouse 13 shows Myka and lists Joanne Kelley in the credits sequence while listing Aaron Ashmore as a guest star. It pretty much guarantees Myka's not gone permanently, and Steve is not going to last.
Subverted, as Myka returns as expected, but Steve still sticks around with the main cast until the end of the series, despite logically speaking, his role as Myka's replacement had been made redundant as well as his constant status as a guest star.
Averted in the UK version of Who Wants to Be a Superhero?, where for episode one (which shows the auditions for the show) the silhouettes of the chosen thirteen are seen with question marks.
The Wire had some AMAZING opening credits sequences (due mostly in part to the various renditions of "Way Down in the Hole" over some really jarring montages) that occasionally would drop scenes that you're looking for the entire season (if you watched it all on DVD). While you can't tell it's Stringer's hand unzipping D'Angelo's baby mama in the credits it did seem a little weird when it finally happened.