Mind Your Step

"Bro: "FUCK IM FALLING DOWN ALL THESE STAIRS ................... "

Jeff: "I WARNED YOU ABOUT STAIRS BRO!!!! I TOLD YOU DOG!""

- Sweet Bro and Hella Jeff ]

There is a set of stairs that has one step that has something wrong with it. Perhaps it's broken, perhaps it squeaks. For whatever reason, the characters have reason to avoid it. And, most of the time, they do. Until they one time when they are in a tearing hurry and they forget all about it. Their foot comes down on the defective step and Hilarity Ensues.

May lead to Death By Falling Over, both in serious and funny contexts, and also in real life. This trope is one reason for a Staircase Tumble, but not the only one.

Comic Books

 * In the Tintin album Recap, a broken stair in Marlinspike Hall puts Captain Haddock in a wheelchair after he forgets about it. At the end of the story, when he is out of the wheelchair and the stair is freshly repaired, he forgets about it again, steps on the still setting marble, and sends himself flying, undoing the repair in the process.

Film

 * In Muppet Treasure Island, Benjamina (Ms. Piggy) ruins her huge entrance by tumbling down the stairs. She blames her pet anteater.
 * This happens in Who Framed Roger Rabbit when Eddie takes Droopy's elevator.

Literature

 * In the Harry Potter novels, one of the staircases in Hogwarts has a 'sinking stair' that traps anyone who steps on it. Neville usually forgets about it and gets stuck. It becomes more serious when Harry is trapped by it while wearing the Invisibility Cloak in Goblet of Fire.
 * A more passive example from Harry Potter: when the Weasleys come to pick Harry up from the Dursleys, he warns them (when they sneak down stairs) that one of the stairs creaks.
 * In The Book of Amber, Gerard keeps insisting that there is a loose step on the stairs to the Pattern Room. No one else ever seems to notice it.
 * Gerard is The Big Guy, literally as well as figuratively, so it's possible that the stair only squeaks under his greater weight. He gets annoyed enough by it that he leaves a note pinned to it with a dagger that says "This one, damn it!" but even knowing which one it is, nobody can find anything wrong with it.
 * In The Belgariad, Belgarath has loose step on the stair to his tower. Turns out the reason is that he put a diamond under it, in order to see how long it took for it to be ground to dust. Throughout the books, everyone simply skips that step. At the end, when he's finally repairing the step, he remembers putting the diamond there and why. But because everyone always skipped over the stair, he never will find out how long it would take to grind it down.
 * In A Wrinkle in Time one of the steps on stairs (not stairs on the steps) creaks and Charles Wallace uses that to signal to Meg that he wants a conference.
 * In Sword of Truth, the condition of the stairs in the house which Richard moves into with Nicci is bad enough that he launches into a full-scale tirade at his three thuggish, knife-wielding housemates, saying something along the lines of "put a shirt on and fix those stairs or you'll never amount to anything". Two of them listen, and do end up better for it. The third is another story.

Live Action TV

 * In Muppet special A Muppet Family Christmas, there's a running gag about an icy patch in front of the door. "Careful of the icy patch!" greets each new visitor - after they've already slipped.
 * The British one-off drama Number 27 used this seriously as a kind of Chekhov's Gun. One of the characters was an old lady who constantly complained that her landlord would never repair the broken step on the stairs up to her flat. The landlord himself forgot the broken step, tripped on it and broke his neck.
 * This happened to game show host Jim Peck on The Big Showdown. The show had a large staircase at the back, and on his way down, Peck tripped on one of the stairs. They left the camera running as he got up, looked at the audience and said, "Sure, you people would applaud a lynching!" Definitely a Crowning Moment of Funny.
 * But was this due to a defective step or Peck simply losing his footing?
 * The broken step in the Dunphy house is a Running Gag in Modern Family.
 * A variant in Everybody Loves Raymond. When Ray hurts himself in a particularly athletic bedroom session, he claims he tripped on a broken step to his parents. It backfires when turns out the stairs really were broken and Frank falls THROUGH them when he investigates.

Web Comics

 * Homestuck plays a variation: Tavros is repeatedly warned about stairs, specifically that his new robot legs are not fit for traversing them as of yet. He's got too much self-confidence to heed the warnings, though, and chooses to try going up and down the things to test them out. Much as you'd expect, his legs fail to handle them properly and he trips down them. Repeatedly.
 * Which is is a reference to Sweet Bro and Hella Jeff by the same author.

Western Animation
""And watch out for the third step. It's -" *crunch* "- busted.""
 * Stoked!

"Sylvester: "Watch your step on this bridge. Some of these boards are looooo..." *SPLASH*"
 * This often occurs in classic animated shorts, especially the ones made by Warner Bros., and is usually followed by: "Watch that first step, it's a lulu", or some other variant.
 * Happens almost word-for-word in Goldimouse and the Three Cats when Sylvester takes his family for a walk: