What is the best example of foreshadowing in a book, movie, or TV show?
One great example is in Stephen King's PET SEMATARY. The moment the discrepancy between the two death dates for Jud's dog appeared, I knew Gage was doomed. The existence of two different death dates for the dog hints to the reader that the Pet Sematary has the power to restore the dead to life. Since the protagonist (Louis) must face the highest possible stakes to impel him to risk this power, we know Gage has to die. And King makes no secret of Gage's forthcoming death - it's mentioned in so many words during the kite-flying scene. I still think this is one of King's all-time best novels.
Well, it's not the best example but a great example of foreshadowing as a plot device.
The first time, Tony Stark tests out his suit, he forgets the temperature change in high altitude and his suit begins to freeze up literally. At the end of the movie, with his suit running out of power, when chased by Iron Monger, he makes the strange decision to go straight up with Monger saying, I can catch up to you and Tony replies, "did you solve the icing issue?" and the Iron Monger losing power as his suit freezes up.