If Covers Always Lie, trailers can, too. Sometimes Tonight Someone Dies or hyping The Reveal might not be enough. And with the Internet an open window these days for writers and directors to viewers' likes, dislikes, hopes, predictions, and Shipping loyalties, it's easy to know exactly how to bait fans into watching the next episode. Be careful not to believe everything you see, though, because as all Fan Vid makers know, any scene can be mixed-and-matched with another to look completely different from their real context.
Of course, Tropes Are Not Bad, as used correctly this can be a very clever way of averting Trailers Always Spoil (by not mentioning or downplaying the real main plot) or Hype Backlash (by underselling the quality and letting word of mouth do the rest once it actually is out). But we're a cynical bunch here at TV Tropes, and in the worst examples it'll actually drive away those who would have otherwise enjoyed it, by completely confusing the relevant demographic.
Indeed, the creation of fake trailers to make a movie look like it's from a completely different genre has become one of the Internet's most beloved recent art forms, such as The Shining as a family-oriented romantic comedy, the one that started it all, The Ten Commandments as a chick flick, or Mary Poppins as a slasher horror flick.
Another way it can backfire is if you can't find enough good footage to make a decent trailer, Genre Savvy audiences can extrapolate just how bad the rest of the material must be.
If it sounds too good to be true, it usually is.
See this list for more examples.
Yes, commercials count now. This trailer for Snickers' Super Bowl ad leads you to believe that the guys in it really have Betty White on their team. In the actual commercial she is called "Mike" and is told, "You play like Betty White!" She then eats a Snickers, and after a brief cutaway, she's replaced by a guy, proving that she only appears in the ad to support the tagline: "You're not you when you're hungry." And so does Abe Vigoda.
In The Prayer Warriors, Chapter 10 of "The Evil Gods Part 2" is titled, ""Piper and Jerry goes to Washington DC to Find out Who the Tractor is and Defeat them Once and for All so they would not terrorized by them ever again for as long as God allows Time to go on For." The Prayer Warriors go to "Washing Dick", but do not identify or defeat the traitor.
At the end of Shadowchasers Twice Told Tales, the author provides a rather vivid and dramatic "trailer" for Shadowchasers Backwater that he admits should not be taken fully at face-value, claiming it contains many scenes where the contexts are probably either misinterpreted and/or incomplete.
When Waiting for Godot made its American debut in Miami, its marketing prominently featured stars Bert Lahr (The Cowardly Lion) and Tom Ewell (from The Seven-Year Itch). Posters declared the play to be "the laugh sensation of two continents."
An egregious example is the trailer for the third Arfenhouse movie. The actual movie turned out to be a few seconds long (not counting credits) and was a Take That to all the creator's fans who were nagging him to deliver on his promise of a sequel, though real sequels were eventually made.
Parodied by Legendary Frog in the One Ring to Rule Them All: Special Edition, in which a "One Ring 3" Teaser includes shout outs to the movie Speed, the Incredible Hulk, and Charlie's Angels. Lampshaded by Sauron, who asks "Will any of this be in the actual movie?" His goblin assistant, Wayne, tells him that it'll all be cut in post-production.
The season 7 trailer for Red vs. Blue showed both Church and Tex watching over the Red and Blue teams in Valhalla. Neither actually appears in that season. In fact, according to Word of God, the "original" Church actually died at the end of the last season, so it seems likely Tex did as well.
The difference here is that Red vs. Blue trailers are more like prelude episodes than actual previews. The season 7 trailer contained no footage from later episodes because it wasn't supposed to. Church and Tex seem to have been inserted into it to encourage Wild Mass Guessing as to what unseen role they may have had in the season.
This ended up backfiring a bit for the initial episodes of RWBY, as early trailers made it look like a fairy tale-themed Dead Fantasy. The first few episodes turned out to be more gradual and comedic with little fight scenes, causing some disappointment among fans. Rooster Teeth admitted they gave a pretty poor showing with that choice a little around the time of the series debut.
If you aren't familiar with Sluggy Freelance then the first half of this video would have you convinced that the comic revolves around... Well a relatively minor side character. After that it breaks down into half a dozen tiny trailers not to be entirely trusted, before finally giving up and summarizing the comic in one sentence.