Your favorite "that man behind the curtain" moment.

In The Wizard of Oz, the Wizard turns out to have more in common with P.T. Barnum than with Harry Potter. (Nicely re-told in the video for Blues Traveler’s Run-around.)

In the first (and only worthwhile) incarnation of Scooby-Doo, the supernatural critters ALWAYS turned out to be human fraudsters.

In Pathfinder, the shaman casts spells against the enemy tribe. But the curses mainly take the form of “Spirits, please guide our enemies into this mechanical deathtrap I am building in their path.”

My favorite is from Son of Sinbad. The lair of the Forty Thieves is in a cave whose door opens when you say “Open, Sesame!” But it’s not magical. The door’s mechanism is powered by a capstan. The capstan is turned by a donkey. The donkey’s name is Sesame.

What’s your favorite?

That is awesome! Thanks for sharing. I don’t have one to share but look forward to others.

Bump.

Surely I’m not the only one who likes the trope. It was common in pulp magazines. Tarzan and Doc Savage had it on multiple occasions. Arthur Conan Doyle and Umberto Eco used it.

Anyone?

“What does God need with a starship?”

The Mandarin in Iron Man III. An excellent way to back a villain out of a racist stereotype and Ben Kingsley was great in both facets of the character.

“The Corbomite Maneuver” in Star Trek is an obvious example.

Open, sarsaparilla? Open, Saskatchewan? Open, saddle soap?

So has anyone seen the latest Spider-Man film? Just asking.

In the classic “Upsadaisium” story arc of Rocky and Bullwinkle, one of the villains is the tall and menacing Mr. Big, who is only shown by his looming shadow. Eventually, it’s discovered that Mr. Big is shorter than Rocky, and uses a flashlight to create the shadow on the wall.

How about a cloaked starship?

That was the first thing I thought of.

There are probably quite a few mystery/detective stories that, like Scooby Doo, involve supposedly supernatural phenomena that turn out to have Perfectly Rational explanations. I think G. K. Chesterton used this trope quite a bit in his mystery stories, but I can’t think of any really good examples right now—maybe “The Hammer of God” counts. And I remember it being used in several of the Three Investigators books by Robert Arthur et al, including the first of the series: The Secret of Terror Castle.

The Hound of the Baskervilles.

“The Blast of the Book” in G. K. Chesterton’s The Innocence of Father Brown. (If I’m remembering which story correctly.) A used book dealer comes into possession of a book said to have magical powers. His employee, left alone, opens the book and apparently vanishes. The book dealer is frightened to have the book around. Father Brown arrives and quickly shows him that the book isn’t magical and the employee was just pulling a prank.

Likewise The Fear in the original Twilight Zone, in which apparently giant aliens turn out to be balloons put up to scare humans by tiny ones.

I think this qualifies:

When I was a kid, I saw an episode of GI Joe that I still remember. The Joes got a call from somebody calling himself “The Viper”, who was warning them that he was coming for them. The whole episode was basically the good guys getting ready for war.

At the end of the episode, an old man shows up. He holds a squeegee. He announces, “I am the viper. I’m here to vipe your vindows.”

I think it stayed with me because I really thought the writers phoned it in on that one.

I found a summary: The Viper is Coming | Joepedia | Fandom

In Gwar’s epic movie “Ragnorok”, a comet called ragnorok is spotted hurtling towards Earth. They spend half the movie partying like it’s the end of the world. But it turns out (spoilers for those of you who have yet to see the ridiculous and shitty low budget gwar film from last century yet)

[Spoiler]The comet is actually just Cardnial Sin, a giant space robot Pope guy coming to Earth to hunt down and destroy Gwar and harvest their jizmoglobin.

after hijacking a space shuttle to unsuccessfully fight cardinal sin in space, and a long drawn out battle that destroys a city, Gwar is forced to revive their pet crack-addicted T-rex to help finally win the day.[/spoiler]

Going with the size trend…

The episode of Buffy where the fear demon turns out to be itsy bitsy.

The bit from various incarnations of Hitchhikers Guide wherein some careless word (Belgium!) uttered by a human makes its way through some spatial anomaly at just the right moment to start a war. The aliens eventually figure it out and come to attack Earth in a campaign for revenge, only for the entire fleet, not anticipating issues of scale, to be swallowed by a dog.

Oh, yes! I love that one!
Giles Oh, bloody hell. The inscription.

Buffy What’s the matter?

Giles I should’ve translated the Gaelic inscription under the illustration of Gachnar.

Buffy What’s it say?

Giles "Actual size. "

I thought Arthur said something more along the lines of “I’m going to have to rethink my lifestyle.”

Belgium was the word that a woman at the flying cocktail party used egregiously in a screenplay, winning an award (which turned out to be part of the key to the dimensional spacetime prison in which the genocidally xenophobic inhabitants of Krikkit had been cooling their heels for the last few millennia).

IIRC.

They were lazier than you think. That was an old joke I heard in summer camp in the early 60s.