Genuinely clever foreshadowing

When Dana puts her groceries on the counter in the start of Ghostbusters, there is a bag of Stay-Puft marshmallows complete with Mr. Stay-Puft.

Does this mean there might be a reason why he mispronounces Chianti then? He says key-AN-tee instead of key-AHN-tee.

It’s a British thing, so I always assume it was a slip on his part.

Another bit of Pratchett brilliance: At the beginning of Hogfather, the children are scared by a monster and Susan uses the poker from the fireplace to beat it up and send it on its way. She then reassures the children that the poker kills monsters, and only monsters.

It will do more monster-killing before the story is finished.

From Firefly:

In “Serenity” (the original two-part pilot episode), Kaylee (the ship’s mechanic) says she’d like to find a “brand new compression coil” for the ship. Mal (the captain) replies that “And I’d like to be king of all Londinum and wear a shiny hat” and tells her they need to find some paying passengers. Kaylee notes that if the coil busts they’ll be “drifting”. Mal says “Best not bust, then”.

In “The Train Job” (the second one-episode pilot, after the network decided not to air the first two-episode pilot for some gorram reason), Mal complains to Kaylee about some non-standard re-wiring in the engine room (“And Kaylee, what the hell’s goin’ on in the engine room? Were there monkeys? Some terrifying space monkeys maybe got loose?”). Kaylee replies that she had to rewire everything because “somebody won’t replace that crappy compression coil”.

Finally, in episode 8, “Out of Gas”, an explosion caused by the “catalyzer on the port compression coil” leaves the the ship stranded in the middle of nowhere with no engines or life support and everybody nearly dies.

(And in a bit of “aftershadowing”, in the next episode, “Ariel”, they’re in a junkyard, and the ship’s pilot, Wash, picks up a piece of junk, then throws it away with an “Oh, great, one of those” expression–which is clearly the same part shown in “Out of Gas”.)

Wonderful example of this in A Drowned Maiden’s Hair, by Laura Amy Schlitz. And you can’t foreshadow a key plot point much earlier than she does in that.

In Moulin Rouge, when they pitch the show to the Duke, they also give away the entire plot of the movie:

“The courtesan and sitarman
are pulled apart by an evil plan
But in the end she hears his song
And their love is just too strong…”

“…And in the end should someone die?”

Fringe had a habit of slipping in cameos of Observers all over the place even before they became a part of the plot. I recently was watching a rerun of the first episode on Sci and even caught a glimpse of one there.

I just finished watching that, and the clues and foreshadowing are everywhere if you know what the secret is (or for both secrets, actually - consider the fate of the doves and what happens later with Angier). The film is riddled with them, right from the beginning.