In an Unintentional Period Piece, references to popular fads or news stories at the time a work was created may be unfamiliar to people viewing the work during the present day.
However, there are some works that feature a reference to an element of its own production; this element was part of the work's initial release, but no longer exists during later releases. Other works may tie into a separate work that existed at the time but has since been forgotten. These are Production-Related Period Pieces, in which these references may seem ambiguous or confusing to people who are unfamiliar with the work's original release.
For example, say an episode of a television series crosses over with an episode from another series, but one of the series became much more popular than the other and is far better remembered. If the less popular series no longer airs and has since been forgotten, then the crossover no longer makes sense to the general public and can lead people to ask What Happened to the Mouse?.
- During Wolverine's 35th anniversary in 2009, Marvel Comics issued variant covers featuring him for all of their titles during the month of April. This was even done for books like Anita Blake: The Laughing Corpse, an adaptation of a novel which did not feature Wolverine in the story and doesn't even take place in the Marvel Universe. Today, fans of the Anita Blake character may wonder what Wolverine is doing on the cover of one of her issues.
- Can happen when a comic book run gets reprinted without all the issues related to a particular story.
- Early reprints of Alan Moore's Swamp Thing run in The '80s began with "The Anatomy Lesson", the second story Moore scripted for the title, which begins with the title character already captured by the Sunderland Corporation. Beginning in 2009, Moore's first issue was included in reprints as well; however, even though that story details Swamp Thing's capture, it also continued the preceding storyline by writer Martin Pasko, which would not be reprinted until 2017.
- In Young Justice, Secret's first appearance in the series references events from her less-known debut one-shot, where the boys help her escape from the DEO. This one-shot went un-reprinted for many years, but was included when DC finally began releasing YJ omnibuses in the new tens.
- Prevalent with Crisis Crossovers, where the main series crosses over into other titles. The Trope Namer Crisis on Infinite Earths is one example, with a number of subplots expanding out of the original series and into the crossover issues. Fortunately, a number of these were footnoted so fans would know where to look to find their resolution, and by 2019 all of the crossover issues were reprinted by DC in separate collections.
- Next Men includes the characters Donna Wojciechowski and Walker Stone from creator John Byrne's then-recent prose novel Whipping Boy. A footnote even directs readers to pick up the novel to find out about "the business in Faulkner" that Stone refers to. Unfortunately, this novel hasn't been in print in about 30 years.
- The 1960s Superman story "The Night of March 31st" contained a number of odd occurrences happening to the Man of Steel. The final panel directs readers to the explanation, where it's revealed that the entire story was an April Fools' Day gag, as well as part of a contest where readers could count the errors in the story for a chance to win prizes. However, when reprinted in the 1980s collection The Greatest Superman Stories Ever Told, the explanation was omitted.
- When re-runs of America's Funniest Home Videos announce an Assignment America contest or a Grand Prize vacation, it is a contest that would have been new at the time the episode was originally broadcast. This is why it's common for the show to throw up an on-screen message saying "This contest has ended" during that portion.
- Averted by Diff'rent Strokes. The series had three crossovers with the short-lived sitcom Hello, Larry during its run. In syndication and on DVD, the Hello, Larry episodes were included.
- In the Doom Patrol (2019) episode "Dumb Patrol", the team is briefly reunited with their one-time nemesis Beard Hunter, who was last seen getting banished to the White Space with Mr. Nobody. When they ask him what happened to Mr. Nobody, he remarks that he hasn't seen him since he got hired for "some animated bullcrap", at which point an ad for Harley Quinn (2019) (in which Mr. Nobody actor Alan Tudyk voices The Joker) pops up on the screen. This ad doesn't appear in all versions of the episode, so some viewers might not get the joke.
- The Drew Carey Show had a yearly April Fools contest where the April Fools episode would be loaded with deliberate errors and viewers were encouraged to send in lists of every error they found, with one lucky and observant viewer being rewarded. As a consequence, every season, there would be an episode where the plot would grind to a halt to announce who won that year's contest.
- Fraggle Rock: In 1983, spots ran on HBO for a sweepstakes where the grand prize was a trip for four to see the set of Fraggle Rock and a free full year subscription to HBO. The same television promo is included on the North American DVD releases of the show but with an on-screen message that the contest ended on May 31, 1983.
- Game shows in general suffer from this in reruns. There might be references to long-ended contests, numbers to dial if you wanted to be a contestant, etc. Depending on the show, these might be left alone, or edited to remove or hide the outdated stuff.
- Wheel of Fortune, for instance, had a promotion tying in with the 1996 Summer Olympics in Atlanta (where home viewers could submit puzzles for the show to use); GSN reruns edited all this stuff out, leading to lots of abrupt fade-outs and the like (most of the Olympics-related episodes were absent from GSN, likely due to this).
- The George Lopez Show:
- The episode "George Goes to Disneyland" has many "Hidden Mickeys" hidden in the background throughout, and includes a Cold Open where the stars break character to explain it's part of a contest to win a Disneyland Resort vacation. It's for this reason that the episode aired only once and would be barred from syndication (it would be released on DVD and iTunes, however).
- The episode "George Gets Cross Over Freddie" is Part 1 of a crossover with Freddie, with Part 2 airing on said show immediately after on their original ABC airings. However, Freddie would only last one season, while George Lopez would get syndicated and become more well-known. And unlike the above-mentioned episode, this episode wound up in the syndie package, which would lead to confusion about a plot that would apparently be abandoned and forgotten about in the next episode in rotation.
- Near the end of The Golden Girls episode "Yokel Hero", Harry Weston (from the spinoff Empty Nest) comes over to bring the girls their mail when Blanche proceeds to come on to him; she later comments to the other girls that Harry is finally available for her to date after the death of his wife. This subplot then continues in the Empty Nest episode "Fatal Attraction", which aired the same night as "Yokel Hero" during its original airing. However, since Empty Nest is far less popular than The Golden Girls today,note the scene from "Yokel Hero" appears to lead nowhere in the series itself and seems unnecessary.
- Zigzagged with In Living Color! for their live Super Bowl halftime episode from 1992. All of the elements related to the Super Bowl are explained within the show itself, including the countdown clock telling viewers when halftime was over so they could switch back to the game. However, viewers of the episode today may wonder why In Living Color thought they could compete with the Super Bowl halftime show when it contains massive stars and is almost as hyped as the game itself. The reason was, at the time it aired, the halftime show WASN'T as popular as it eventually became, and its entertainment more often than not led viewers to want to take a bathroom break instead. After ILC's gimmick in 1992, the halftime entertainment for the following Super Bowl was none other than Michael Jackson, and the show was taken much more seriously ever since.
- One episode of Law & Order: Criminal Intent, in which the team investigates the death of a former hitman in witness protection, featured a brief guest appearance by Marshal Mary Shepard, the protagonist of In Plain Sight, who gives them a briefing about the victim before saying that she has to run so that she can be back in Albuquerque by nine, which causes Mike Logan to ask why anyone would need to be in that city by that particular time. Nine o'clock at night was IPS' original timeslot.
- The Married... with Children episode "Assault and Batteries" was originally broadcast in 3D. As such, it makes a number of jokes about 3D throughout the episode. The fact that it was aired in 3D isn't indicated during syndication airings or (possibly) the DVD releases, so the jokes during the episode no longer make sense.
- Mystery Science Theater 3000: The episode "Night of the Blood Beast" was initially a Thanksgiving Episode that debuted as the finale of the Turkey Day '95 marathon. The marathon included sketches between the episodes, with Dr. Clayton Forrester desperately preparing a Thanksgiving party for a bunch of unwanted guests. This plot continued into "Night of the Blood Beast" itself, where the host segments showed Forrester's party begin in earnest. The marathon-specific sketches were never aired again, so Best Brains created an alternate standalone version of "Night of the Blood Beast" for reruns, with new host segments that made absolutely no reference to Thanksgiving. The eventual DVD release included both versions of the episode.
- Red Dwarf: The initial VHS releases of the Blooper compliations Smeg Ups and Smeg Outs included continuity links for two competitions. (Smeg Ups had one to allow the winner the chance to become an extra for Series VII, whilst Smeg Outs had the chance to allow two people a day on the set for the same series). This naturally would not have made sense to any viewers viewing them after the competition ended, so when they were rereleased on DVD (as Just The Smegs), these competition links were cut.
- The cold open of the Saturday Night Live episode hosted by Jennifer Lopez in 2001 features Will Ferrell apologizing to Lopez for his treatment of her earlier in the week. What is this in reference to? A sketch from a special prime-time episode of SNL which aired the previous Thursday night, which probably hasn't been repeated since its original airing (unlike the regular episode hosted by Lopez).
- In 1997, ABC aired a special called The T.G.I.F. Time Warp which featured the casts of Sabrina the Teenage Witch, Boy Meets World, Teen Angel and You Wish stuck in different time periods due to Salem swallowing a time ball. Anyone watching the latter three shows without knowing about the special would be quite confused when their main casts are sent to different time periods for seemingly no reason. Fortuitously for Boy Meets World (the only other show besides Sabrina to survive more than one season and be seen in syndication), Salem appears only in the Cold Opennote before abruptly disappearing (Cory literally tosses him aside), and the show had a tradition of fantasy episodes set in different time periods anyways, so the Time Warp one fits right in. Sabrina herself had even cameoed in a different episode of the show, so why wouldn't Salem randomly show up?
- This can apply to concept albums, where a hit single from the album will be much more popular over the years than the album itself. One example is "Mr. Roboto" by Styx, which tells part of the story of Robert Orin Charles Kilroy from the album Kilroy Was Here. However, the lyrics make less sense than they're supposed to without the rest of the songs and the liner notes from the album to explain them.
- The 2011 Eminem and B.o.B collaboration "Things Get Worse" is a reworking of a track intended for Eminem's scrapped 2009 album Relapse 2, which B.o.B added a guest verse to and put on his album. The problem is that Relapse was a Concept Album in which Eminem adopts a Medical Horror Slasher Movie persona that Eminem scrapped out of embarassment almost immediately as soon as the album came out. By the time "Things Get Worse" was released, Eminem had changed to a softer Sincerity Mode image and had a Career Resurrection with it - even having a major hit with B.o.B during this artistic period in his new stadium pop style. It is difficult to imagine the audience of Eminem and B.o.B's uplifting "Airplanes" picking up "Things Get Worse" and having any idea why Slim Shady is rapping in a weird cod-Egyptian accent about murdering Dakota Fanning and dancing in her panties.
- This got Eminem into some serious trouble when, in 2019, the solo version of "Things Get Worse" leaked, complete with a lyric in which Slim announces he stands with Chris Brown, saying he'd also beat up a woman who gave him a venereal disease. While it's reasonable to be appalled by the joke, with Eminem's Relapse phase being largely forgotten about in the general population by 2019, it was reported on as if it was Eminem's actual opinions. Eminem put out a statement explaining that when he wrote the song he was getting 'way too into' playing his serial killer character and regretted making the song and any hurt he caused, but it was taken by some as a contrived excuse, despite the fact that the line is typical of the content on Relapse and also said in a silly accent just to reinforce that it's a character.
- Cartoon Network Invaded, a five-part crossover event in which five Cartoon Network shows were invaded by the same aliens, averted this for the most part, since the episodes aired separately rather than together. However, Part 3, an episode of My Gym Partner's a Monkey, contained a reference to the moment in Part 2 where Windsor holds up a magazine with the Eds on the cover. In the case of Part 5, which aired on The Grim Adventures of Billy & Mandy, its original ending featured a stinger tying the five parts together, so an alternate ending was created for when it aired by itself.
- On the topic of The Grim Adventures of Billy & Mandy, the show used to be referred to as "Grim and Evil"note . As The Grim Adventures of Billy and Mandy got more popular, Evil Con Carne was dropped, allowing the series to have 11-minute stories that couldn't work with the 7-minute timeframe. Eventually, ECC's General Skarr moved to Endsville and became a recurring character, with occasional appearances from other ECC characters. Younger fans assume that aside from Skarr, the characters were one-off characters created for the series.
- The Simpsons: In "A Star Is Burns", there is a crossover with Jay Sherman from the short-lived show The Critic; viewers born after The '90s might think that Jay was a one-off Simpsons character from an in-universe TV show.
- In 1996, USA Network held an event where a made-up barbarian character named the "Warrior King" appeared in one episode of each of the shows on the network's Action Extreme Team block (in order of appearance, Street Fighter, Savage Dragon, Mortal Kombat: Defenders of the Realm and Wing Commander Academy), in a storyline where he tried to recover a mystical orb needed to prevent the impending doom of his home dimension. Without the context of the event, the orb's appearance as a MacGuffin in the Mortal Kombat episode and the Warrior King's cameo at the end of it might be confusing to newer viewers.