Movie science 101.

[SynapticBoomstick]

Whole populations of creatures can survive for millenia underground near Earth's mantle without being cooked and without having a renewable source of oxygen or food other than themselves for the latter, though they seem to know when to stop before they end up starving. They can also function perfectly well underground in their original above-ground forms without any difficulty. Any humans or humanoids will also have incredibly huge eyes to catch the staggering abundance of light that one finds miles beneath the ground. There are also apparently clouds down there, too.

(Take your pick of films like that, you'll probably find one of the above to be true.)

[WildHoosier09]

Classic Matrix science: humans generate a net increase in energy so much so that entire machine cities can be powered by humans being fed nothing more than IV of dead humans (hint, humans convert stored chemical energy of food to kinetic/electric energy but can not create new energy, without some kind of new energy coming into this system the humans would die and the machines would power down). 

[BTM]

We could probably do an entire just on the subject on Nuclear bomb, waste and power, and all the crazy stuff we see in movies about that.  But I'll do a light one.

Nuclear waste looks like green liquid!  And it's usually stored in suspicious looking and easily breakable barrels.  Also, if you're evil, radiation gives you hideous looking scars and makes you deformed or, if you're an animal, it'll make you large and increasing hostile.  If you're good, it'll give you superpowers!  If you're dead, it'll bring you back to life as a zombie.

[AndyC]

Atomic power works just like electricity, except with much more kick.

Nuclear weapons temporarily transform their surroundings into the Nevada desert or a South Pacific atoll when they explode.

You can explode sunlight, and doing so will destroy the universe.

Rocket engines in space produce a lot of smoke. Even in deep space, the smoke rises relative to the rocket.

Rockets often look different immediately after launch. Tricks of the atmosphere make them look like either a Mercury Redstone, a Saturn 1B or a Saturn 5.

The human fetus goes through the entire process of evolution before it's born. Mercury will create a chimera (or a big, deformed bear).

Reanimating or prolonging the lives of people will make them evil and ultimately homicidal. The effect is virtually inevitable if you just save the head or brain.

The human heart is a single cell.

[Newt]

The human fetus goes through the entire process of evolution before it's born.

This is a good one - a 'classic' example of running with the popular understanding of a real scientific concept: "ontogeny recapitualtes phylogeny".  Those are the most fun!   

[Cthulhu]

Contrary to popular belief, you CAN outrun wind.
Corpses (zombies) only rot when convenient.

[AndyC]

The human fetus goes through the entire process of evolution before it's born.

This is a good one - a 'classic' example of running with the popular understanding of a real scientific concept: "ontogeny recapitualtes phylogeny".  Those are the most fun!   

I like the ones where a real scientific principle has been screwed up almost beyond recognition. The single-celled heart from The Amazing Colossal Man, for example. I remember from high school biology that cardiac muscle does have a unique structure, with cells branching into each other, and the cells are organized to better squeeze in unison. The writers had facts, they just didn't understand the facts at all.