Stay on the Path

The heroes are journeying through some dangerous area—nearly always a dark and mysterious forest. A wizard or other such guide has sent them on their way with one piece of advice: Stay on the Path. Straying from the path will lead to... well, it's often not quite specified.

The heroes will nearly always leave the path at some point. Expect danger to ensue; they may also find that Short Cuts Make Long Delays. If they truly have to come back to the path to progress, see Yellow Brick Road.

If the heroes leave the path because they were told not to leave it, it's also a case of Forbidden Fruit.

Examples of Stay on the Path include:

Comic Books

One JMS Spider-Man story involves Doctor Strange sending Peter to the Astral Plane to fight a villain, with a warning not to leave the designated path. He of course strays off after seeing some random giant spider-shaped thingie, and accidentally unleashes a spider-hating Wasp spirit named Shathra who makes his life miserable for a while afterward.
From The Sandman.

"I have spoken to the lord of this realm. He has given you permission to enter the castle, and will grant you audience. I am honour-bound to warn you to stay on the path through the castle. Straying from the path could mean your destruction. You killed my friend, woman. Stray from your path."

In the original Books of Magic, Tim Hunter and Doctor Occult take a trip into Fairyland. Occult gives the warning; Tim actually stays on the path fairly well, until Baba Yaga uses an illusion of the Doctor to convince him to step off it. And then tries to eat him.

Fairy Tales

Little Red Riding Hood

Film

The hero of An American Werewolf in London and his wing-man are given the warning "Stay on the road, and beware of the moon!" They ignore the first, and don't understand the second. As a result, one of them dies, and the other becomes a werewolf.
The Golden Child: Before undergoing a test, Eddie Murphy's character is warned to (among other things) "Stay on the path," During the test, the path he's on suddenly explodes, and he angrily demands to know what's going on. A voice tells him that he has to know when to break the rules (an important clue for completing the Secret Test of Character).
The Company of Wolves: Granny tells Rosaleen not to stray from the path. She does and finds a werewolf.

Literature

The Pilgrims Progress used this as a metaphor for staying faithful to Christianity. Of course, the characters constantly persist in straying off the path.
Rowan of Rin had this happen. They were in a swamp that through some means conjured up images of loved ones drowning just off the path. Of course, if you rush off to save them....
The Hobbit has Gandalf give this advice. Inverted in The Lord of the Rings, however, when Gandalf tells Frodo and Sam to stay off the roads and meet up with the Nazgul as a result of Merry and Pippin not following the rule.
The original "Little Red Riding Hood" has the heroine's mother tell her not to stray from the path for any reason. She is lured off the path by the wolf.
Stardust: Neil Gaiman is a fan of this trope. Anyone who strays from the path through the Serewood will fall victim to murderous plants. This trope also shows up in The Books of Magic, during the journey through Fairyland. And in The Sandman, Morpheus warns his guests that he will summon each in turn, and when they are summoned they are to follow the path and not stray from it. If they do, then Morpheus cannot guarantee their safety. One of the guests strays from the path, and ends up creating his personal nemesis...
Ray Bradbury's A Sound of Thunder justifies this trope: leaving the path while on a hunting trip in the past could lead to Temporal Mutability problems a la Butterfly of Doom. A character messes up, leaves the path and steps on a butterfly. Dystopia ensues.
In Harry Potter and the Philosopher's Stone, Hagrid warns Harry not to leave the path in the Forbidden Forest.
David from The Book of Lost Things is warned to stay off the road; when he strays to pick an apple, he is captured by a hunter.
In Dream Park, visitors at the Chamber of Horrors attraction are warned to stay on the green glowing path, as they pass through a holographic diorama of underwater terrors. One woman steps off the path anyway, and is instantly snatched away and torn to pieces by a shark and an aquatic zombie! Lucky for Dream Park's liability insurance, she turns out to be a hologram too.
One of the main themes in the first book of the The Edge Chronicles, mainly because it's set in the Deepwoods, where Everything Is Trying to Kill You. This mentality even extends to children's sports in the woodtroll villages - when protagonist Twig "strays from the path" during a game, he's beaten up by the other kids even though there's technically no rule against it. No-one strays from the path.
In the Dragaera universe, if you're in the world of the dead and you leave the path, you'll never be able to find it again, wandering for all eternity even if you're very much alive.

Live-Action TV

Babylon 5:
- Susan Ivanova is warned not to stray from the path. She disobeys, and is nearly mind-raped by The Shadows.
- It is a really bad thing when you lose the signals of hyperspace beacons.
The Invaders: The old TV series began like this, as the protagonist took a wrong turn somewhere and accidentally discovered the alien invasion going on...

Tabletop Games

Dungeons and Dragons
- Module I5 Lost Tomb of Martek. Before entering the Crystal Prism the PCs read runes that say "Walk that path straight and narrow". If they don't do so, the Iron Phoenix's powerful attacks will probably destroy them.
- Module I12 Egg of the Phoenix. In one section of the adventure the PC's are told "If a path is given, do not stray from it". They really need to follow this advice. If they don't, not only will they not be able to complete the mission and possibly get lost 200 million years in the past, but if they stray off the Platinum Path in the Black Forest they'll end up getting massively energy drained and almost certainly die.
- Subverted in the adventure Tomb of Horrors, where following the mosaic path on a hallway's floor will lead you past a couple of pit traps, then directly into another one. Not that anyone who so much as considers tackling the Tomb of Horrors has any excuse for not testing the floor (and the walls, the ceiling, the air, the doors, the doorlatches...)
Changeling: The Lost makes it very clear that wandering off the path in The Hedge can be a very, very bad thing. Not only do the Thorns that make up most of the Hedge drink a Changeling's power (or a human's soul, but hobgoblins lurk in some of the deeper parts... and of course, the Gentry occasionally take their strolls through it as well.
- Even the predecessor, Changeling: The Dreaming, had elements of this, despite being Lighter and Softer. Wandering off the Shining Path while wandering the Dreaming was a very good way to get lost in some of the deeper delusions of the collective unconsciousness.

Video Games

As alluded to in the trope description, you have to stay on the path in order to progress through The Lost Woods in many games.
Racing games with open areas will often respawn you should you leave the area, for obvious reasons.
This is the idea of the game The Path: Essentially, the player must deliberately lose by straying from the path, thus guaranteeing your character's death while experiencing the more interesting aspects of the story.
The Legend of Zelda Ocarina of Time
- The Haunted Wasteland forces you to follow the flags; going off-route will cause you to be swallowed by the desert winds (and teleported back to a checkpoint). A similar puzzle occurs near Pinnacle Rock in the western ocean of Majoras Mask.
- Zelda II the Adventure of Link: Staying on roads neutralizes enemy encounters. While you're on it, no encounters pop up, and if you spawn some then move back onto the path, the "battle" is a barren plain devoid of enemies you can just walk out of. Naturally, there are many places you can't get to by staying on the path. Early in the game, you'll want these encounters (free Experience Points) but as the game begins to go from Nintendo Hard to "Miyamoto, you sadist!" they become great ways to die horribly (and naturally, at this point, much harder to avoid.)
World of Warcraft: One of the loading screen tips is that players are much less likely to encounter monsters when following a path. Generally true, but there are also some regions where particularly deadly enemies follow the roads. Pre-expanion Rexxar followed a very long path through Durotar and Feralas, and was death to any Alliance character who didn't see him coming in time.
In Neverwinter Nights, a village has been suspended out of time by Lathander. When you get to the castle you have 3 paths to choose from. If you stay on the correct path, then you get a note from Lathander complementing you on your choice of the middle balanced way. The others you get attacked by deamon or devils and appropriately chastied by Lathander.
In Black Sigil, you must cross a desert. You are told to follow the path (marked by specific plants), else you will encounter Sand Worms, which are rather tough for you at that point. Of course, straying from the path also means interesting items.
In Fallout: New Vegas, low-level characters can do quite a bit of exploring in relative safety if they stick to the crumbling roads. Wander just a little way off the path, though, and the restored-to-their-former-magnificence Deathclaws and their adorable new sidekicks the Cazadors await you.
In Final Fantasy V, a moogle shows you the way to the moogle village. He walks into a patch of desert, shakes his head "no", and avoids the desert all the rest of the way. When you take control, walking on the path will result in no Random Encounters, while the encounters in the desert are, at least for your likely current level, likely to spawn a Boss in Mooks Clothing.
The guards in The Elder Scrolls IV: Oblivion will sometimes advise you to do just this.

Web Comics

In No Rest for The Wicked, Perrault urges that this is much wiser than following a deranged madwoman through The Lost Woods.