Jacob Marley Warning

"Don't let Vox do to you what he did to me."

—Last words of : Ace Hardlight, Ratchet: Deadlocked

Bob meets George. Now, George represents what will become of Bob if he takes a certain path. Unfortunately, George has either become evil or else his life has ended up sucking. George will either warn Bob to not make the mistake he did or else he will encourage Bob to follow in his footsteps. The latter is, of course, more likely to occur when George is evil. Either way, Bob will ultimately decide that he does not want to be the next George.

If Time Travel is involved, George may be Bob's future self, possibly a Future Loser.

Named, of course, for Jacob Marley from Charles Dickens' A Christmas Carol. In the book, Marley represents what will become of Scrooge if he continues to be such a selfish jerk.

Not to be confused with Jacob Marley Apparel.

For when the warning goes opposite way, see Fling a Light Into the Future.

Examples of Jacob Marley Warning include:

Anime and Manga

Doraemon: It's the eponymous robot's raison d'etre to stop Nobita from becoming a Future Loser, but occasionally, Future Nobita himself appears to whip his younger self into studying diligently. These are some of the most unsettling episodes, since Future Nobita knows his past-self's tricks and possess Doraemon's future gadgets.

Comic Books

The plot of the DC Comics' comedy miniseries A. Bizarro. Al Bizarro, duplicated not from Superman, but from a LexCorp employee called Albert Beezer, initially thinks he should try to be like Beezer, but at the end of the first issue, Beezer tells Bizarro that his life sucks and in order to be happy Bizarro should "do the exact opposite of everything I did!" (a Mythology Gag on the pre-Crisis Bizarro Code).
Lou Martin, The Slacker protagonist of Major Bummer gets one of these from his time-traveling future self, who regrets having wasted his life. By the end of the story, present-day-Lou acts like he's had an epiphany, but the next scene shows him sleeping in and actively avoiding superheroics as usual.

Film

In the original Star Wars trilogy, Darth Vader represents what would become of Luke if he turned to the Dark Side. This is made most clear when Luke battles an apparition of Vader on Dagobah which turns out to have his face. Later on, Luke nearly kills Vader, but stops himself when he realizes that he is just one step away from becoming the next Vader. The Emperor helpfully points this out just to make sure Luke is absolutely clear on what choice he would be making.
- Even more recently The Force Unleashed's centralman is another representation of what Luke (and potentially Leia) would have been like if raised by Darth Vader.
- Star Wars Expanded Universe comics had an arc where the Emperor Reborn had Luke fall to the Dark Side and join him, apparently to try and limit what he could do, and indeed, the Emperor Reborn dressed Luke like his father, as seen in the pic for In the Blood. And then Leia redeemed him, he tried and failed to kill the Emperor Reborn, and two volumes later finally managed it.
Done with Neo Nazism in American History X.
Done in The Muppet Christmas Carol, of course. Worth mentioning because A) they give Jacob a brother, Robert, so they can give the role of Marley to both Statler and Waldorf, which allows them to B) sing a cool little song about it all. And C) you get "Bob Marley". (Groan)
Disney's The Kid is a rather weird take on this trope...starting with the fact that it's told from the Jacob Marley's perspective.
The 1972 film version of Tales From The Crypt has a rather grim subversion of this. The Crypt-Keeper shows a group of strangers visions of the hideous fates awaiting each of them as a result of various wicked actions; at the end of the film, he reveals that these visions are not warnings of what might happen, but records of what has already happened; they're already dead, and destined for Hell.
Repo! The Genetic Opera: Blind Mag is a more tragic variant on this- she's a completely noble, compassionate person whose only mistakes were trusting entirely the wrong people. She warns her similarly trapped goddaughter Shilo to bolt for her freedom and think for herself.
Coraline has the title character refusing to let the Other Mother sew buttons in her eyes when she asks her to do so, even though the Other Mother had treated her with adoration up until that point. It turns out that she made an excellent choice by not doing so. It turned out that the Other Mother had invited three children to her realm before her after seeing what was wrong in their lives, as she did with the title character herself. Only in their case they had decided to allow her to sew the buttons in their eyes...which allowed the Other Mother to claim their souls for herself to feast upon. Fortunately, Coraline is smart and not only escapes, but throws away the key so that nobody can enter the Other Mother's realm again.

Literature

Charles Dickens must have liked this one. In Great Expectations, Pip can become either Jaggers (the bad choice) or Herbert (the good choice).
Henry James' story The Jolly Corner is a ghost story where a man is haunted by the man he might have become. It's not obvious whether the ghost represents bad choices, though another character remarks that he seems to have suffered more than the protagonist.
In Frankenstein, Victor serves as a warning to Walton, who is in danger of becoming as obsessed with his exploration as Victor was with the science that led to the creation of his monster.
- Though inverted in that he blasts Walton's crew for not having the balls to carry on on Walton's journey. Victor might represent what Walton could become, but unlike other examples he doesn't realize this, or regard it as a bad thing. He blames the Creature for all his ills but doesn't take responsibility for the hell he put the Creature through For Science!.

Live Action TV

In the early Stargate SG-1 episode "The Torment of Tantalus", Dr. Ernest Littlefield represents what would become of Daniel if he stayed on the planet and tried to figure out the Meaning Of Life Stuff.
- In the season ten episode "The Road Not Taken," Cameron tells Sam that she'll share her fate if she doesn't follow Landry's demands. Subverted, as Sam speaks out against Landry anyway.
In Thirty Rock episode "Rosemary's Baby", Rosemary Howard represents what would become of Liz if she left TGS and wrote films based on the premise that art should be offensive.
An episode of That '70s Show featured Eric meeting a 35-year-old who had no job and was as big a nerd as he was. He eventually decided to avoid that fate after he found out the man still lived with his mother.
The title character of Malcolm in the Middle met a 30-something loser who couldn't hold a job, was badly in debt to unsavory types and spent all day trying to get people to play chess with him at the park. He acted in exactly the same way as Malcolm.
Foreman is afraid of becoming like the titular character of House.
The Law & Order: SVU episode "Redemption" features an older detective who represents Stabler if he follows his current path.
Similar to the above The Shield, Vic meets up with his former mentor (played by Carl "Action Jackson" Weathers), who's a walking cautionary tale of Vic's current path (with gangbanger subbing for Armenian mobsters)
- For the record, that's in the first or second season. The show lasts for seven.
In the Friends episode "The One Where Heckles Dies", Chandler discovers that cranky old Mr. Heckles was once a fun guy, and becomes obsessed with the notion that he may become just like Heckles when he gets older.
In an episode of Will and Grace, the titular characters meet an author who Will admires. They discover he's a miserable jerk who spends all day in his house arguing with his female best friend and roommate who's just as big of a jerk as he is. Guess who they remind the main characters of?
Done in Buffy the Vampire Slayer where Xander meets his future self who tells him he will be unhappy marrying Anya. Subverted when it turns out future Xander is actually one of Anya's victims pretending to be Xander.
In Supernatural, Dean gets this warning from the post-apocalyptic version of himself, who implores him not to resist the angels' plan for him. Essentially, he's given the choice of either becoming Michael's vessel or killing Sam before Sam becomes Lucifer's vessel. Predictably, he tries taking the third option of patching things up with his brother so that Sam'll be able to resist Lucifer.
Max Ryan from the Criminal Minds episode "Unfinished Business" represents what could happen to any of the main characters if they allow their work at the BAU to dominate their lives: he's divorced, rarely sees his children and went as far as to move house in order to be closer to one of the crime scenes in a serial killer case that he never solved. Later in the show, Agent Rossi serves a similar purpose, having been divorced three times, being obsessed with an unsolved double murder, having been too focused on fame and having let his whole life pass him by while he worked at the BAU.

Music

The Irish folk song "Paddy's Lamentation"

Here's you boys, now take my advice
To America I'll have ye's not be going
There is nothing here but war, where the murderin' cannons roar
And I wish I was at home in dear old Dublin

- The song is even more poignant once the Fridge Horror sinks in: Paddy is dead and is giving his warning as a ghost.
"House of the rising sun" has (in one of the various versions) the verse

Go tell my baby sister
Never do like I have done
To shun that house in New Orleans
They call the Rising Sun.

Religion & Mythology

In the Gospel according to Luke there is the parable of the rich man and Lazarus (not to be confused with the other Lazarus). The rich man goes to hell and then begs Abraham to send Lazarus back to act as the Jacob Marley Warning to his brothers. Abraham tells him it would do no good, "If they would not listen to Moses and the prophets, they would not listen even to a man come back from the dead."

Theatre

In The Little Foxes, Birdie warns her niece-in-law Alexandra: "Don't love me. Because in twenty years you'll just be like me. They'll do all the same things to you."

Video Games

Archer of Fate/stay night. When he's not trying to kill his past self, he's anviliciously deconstructing his ideals in an effort to ensure he doesn't make the same mistakes he did. This only comes up in two of the three routes. In the first, he is eventually convinced that, even if it hurts, maybe he didn't make such a big mistake with his life. In the other, after seeing his past self has all but abandoned his ideals, he agrees to help him, committing a Heroic Sacrifice and giving him the power he needs to succeed.
Anders of Dragon Age II delivers a nightmarish warning to Merrill, telling her that if she continues to use Blood Magic, she will become possessed like him - trapped in a body controlled by another being, unable even to scream.

Web Comics

Used once in Narbonic, using a combination of Time Travel and Projected Man.
Parodied in Brawl in the Family during their A Christmas Carol parody arc. Wario takes the role of Marley, who sees nothing wrong with the path Mario is taking, but is sent to warn Mario anyways.
Played with in The Inexplicable Adventures of Bob. Molly's clone, Galatea, was raised under much worse circumstances than Molly, and turned out mean, paranoid, and borderline villainous. When Galatea finally starts to get herself in order and become less of a jerk, Molly's family offers to let her live with them. Galatea makes it clear that she appreciates the offer, but refuses because she's afraid of losing her own identity if she were to start living exactly as Molly does. A subversion, since Molly's life was far nicer and more stable than Galatea's at that point.

Western Animation

Parodied in the "My Future Self 'n' Me" episode of South Park, where an organization hires actors to play kids future' selves to keep them from using drugs. It's also played straight at the end of the episode with Cartman.
- Not quite straight; the Future Cartman is a handsome and successful businessman who says that this is the big turning point in his life, and encourages his younger self to stay on the straight and narrow. Cartman, convinced that the guy is just another actor, resolves to be even more of a selfish jerk than he normally is. After he walks off, the Future Cartman changes into a fat slob.
  - So basically future-Cartman screwed himself over because he absolutely had to tell his younger self where he'd end up, even though it was presumably unnecessary in the first place? ... What an Idiot!.
The Doctor turns out to be a Jacob Marley to Candace in Phineas and Ferb: Candace Across the Universe. While the Doctor isn't wrong to consider herself to be similar to the redhead (even if they're not technically from the same planet), she also represents what Candace could have become if she decided to put an end to Phineas and Ferb's fun... for good.
Kim Possible: This trope happens with Ron and Adrena Lynn. Adrena indirectly explains to him that if he continues to lie to everyone in order to make himself popular, he would end up becoming a hated outcast instead. Especially since Adrena didn't care that people could get hurt trying to replicate the stunts, even if they were children.