When there's one maverick scientist (usually extremely attractive) who predicts an immediate disaster, only to be told "That's impossible!" by every other mainstream scientist, and is believed only by an equally good looking reporter, and they both then go directly to the site of said disaster so that they can barely escape in time!

Of course, the thing they predict usually is impossible, or at least wildly exaggerated. But that's Hollywood.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort