Precap

During the 1970s and 1980s, the increasing popularity of channel surfing led to networks putting mini-trailers for their shows on the start of that very show. Usually preceded by "on tonight's X..." or "this time, on X...", the clips would then show all of the best stunts, most of the guest characters, an overview of the plot and sometimes even reveal major plot twists. As most shows back then weren't exactly complex affairs, this usually meant that the entire plot was summarized in the first two minutes, turning the following runtime into a tedious exercise of filling in the gaps.

Ironically, although the practice was invented to keep people from flicking over to another channel, summarising the entire story in two minutes may have had the unexpected side effect of facilitating channel surfing.

These days this trope is almost exclusively reserved for Reality Shows, and even then only shows the first ten or fifteen minutes. This is still annoying for some, however, since any show that involves members of the cast being voted out at the halfway point can be inadvertently spoiled for those that are paying attention to who's in each scene.

In console games, this is a subtrope of the Attract Mode.

Compare Trailers Always Spoil, Spoiler Opening and "On the Next...".

Examples of Precap include:

Anime and Manga

More recent episodes of Pokémon start with a scene from the middle of the episode, essentially making every episode into a How We Got Here.
Transformers Victory used previews.
Cardcaptors would sometimes have these.
The first season of Rosario + Vampire had a selection of clips from each weeks episode in it's opening.
- As did the second opening of Fate/stay night
Sailor Moon did this up until season four, when it got rid of the precaps and instead used that space for a more tradtional Cold Open.
Bleach has started doing this in the mid 200's.
Vandread had blipvert-style precaps in each episode's opening, similar to the Battlestar Galactica reboot.

Live Action TV

The A-Team had these at the front of the episode with assorted dialog and action sequences put together.
The Amazing Race did this only in its fourteenth season.
Parodied in the S.O.B.s episode of Arrested Development. It's announced at the beginning that someone will die by the end, but halfway through the episode, the narrator reveals who it's going to be.
The original Battlestar Galactica.
The new Battlestar Galactica also did post-teaser Precaps in the opening credits, an homage to the similar rapid-fire Precap credits on Space: 1999. They're very fast and have no dialog, making them relatively spoiler free. Usually.
- And even if there is a spoiler, it doesn't mean what you think it means.
They did this on The Dukes of Hazzard, showing highlights of the evening's dramatic moments and car stunts.
Ellery Queen: "This man is about to be killed. Was it [scenes of various characters saying how much they hated the victim but didn't kill them], or someone else? Match wits with Ellery Queen to see if you can discover Whodunnit."
The Incredible Hulk also featured these, almost always ending with a scene from just before or during the second "Hulk-out".
Knight Rider featured these in many episodes.
Mighty Morphin Power Rangers did these and "On the Next..." for a while.
This one goes back to the 1960s. Each intro to the original Mission: Impossible featured a quick action montage of scenes from that evening's episode over the light fuse animation. A few episodes from their final season also had the traditional teaser format prior to the theme intro.
This practice was commonplace for majority of the NBC Mystery Movies (Columbo, McCloud, McMillan and Wife, etc.).
Police Story showed scenes from the episode of the day as part of the overall intro to the show, which was one of the first "short" themes, which in itself was a forerunner to today's program intros.
Thunderbirds also showed a very quick montage of action scenes from the upcoming episode of that series.
- Gerry Anderson would later use this trick again for Space: 1999 and Space Precinct.
SeaQuest DSV only used precaps on a handful of episodes, where the events were actually major enough to be worth teasing.
Top Gear does this in the first episode of the season, showing previews of all the stuff that is to come that was already filmed for that season. The best stuff is saved for the actual episodes that they appear in.

Video Games

Most of the Resident Evil games feature FMV of various scenes in the game - usually bosses, but sometimes clips showing how to complete some of the puzzles.
Most video games, in fact, if left idle on the main menu, will go into Attract Mode. This may consist of showing a clip of action from the game, either a short snippet of a cut scene, or a gameplay demo.
- Super Metroid for the SNES goes a little farther than just showing gameplay snippets during its "attract mode"; the locations of three suit power-ups and how to get them are shown, as is a special technique that can recharge the Power Suit at the expense of ammo (though the exact method to do this is not shown - only the result, and that it's possible)
  - Those are only shown after you beat the game once, so it's not such a big deal.
The Legend of Zelda: Twilight Princess, if left idle at the outset, will run through two different full-length trailers showing various highlights of game play and cutscenes. The combined length of the two trailers comes to nearly ten minutes.
- The original Legend of Zelda would scroll the (badly translated) backstory of the game if left at the opening screen for too long.
Persona 4 does the same thing as the Twilight Princess example above, if one suffers a game over or chooses to return to the title screen, featuring a remix of the main battle theme over a (spoiler-laden) montage of gameplay.
- Persona 3 FES actually has two different intros - the one from the original game and the one made for FES (which spoils bits of "The Answer"). The two alternate each time you go back to the title screen.
Dead Rising includes a short cutscene about a family crashing their car and being overrun by zombies. It looks unrelated until you see a picture of them in the wallet of the Psycho Vietnam Vet Boss. It was their death that made him crazy.

Western Animation

In at least one instance, Total Drama Island did a preview for next week's all new episode... before this week's all new episode had ended. Being that TDI involves people being voted off, well, now one knew some of the people who were safe. Way to go, TDI.
- TDI also came up with parody clips...of the new episode, during said episode. The point was to be silly, usually along the theme of 'One character loses his mind, scares the others and or believes he is a giraffe'. They'd use clips of stuff that hadn't been shown yet, inducing more spoilering.
Many episodes of The Flintstones had, at least for some airings, a seemingly random scene from the episode run before the episode begins. Later viewers unaccustomed to the practice watching decades-old reruns on Cartoon Network were probably confused to no end by this.