Most of the time we're willing to shovel down the popcorn and watch Yoda lift X-Wings out of the swamp using nothing but the Force and a smattering of questionably parsed English, or let Jean-Luc Picard get the Enterprise out of a scrape by the convenient discovery of yet another type of particle beam. But every once in a while we just have to vent about some of the truly egregious "fiction" in science fiction.
1. Sounds in Space
The tag line from Alien got it right: "In Space, no one can hear you scream". The reason no one can hear you scream is that sound needs air to travel in, and there's none in space.
Most of space is a hard vacuum, with a molecule or two of hydrogen floating around in every cubic meter - not nearly enough to transmit sound. Every sound in the movies, from photon torpedoes and laser beams to exploding starships and hyperspace booms, would never happen in real life.
For that matter, you'd never see laser beams in space either, since in a vacuum there's no medium to reveal them. So a real-life laser dog fight in space would be really boring to watch.
2. Faster-Than-Light Travel
Warp drives and hyperspace are very useful in science fiction, but there's one catch. According to Einstein, the speed of light isn't just a good idea, it's the law. Nothing can go faster than the speed of light in a vacuum (that's about 186,000 miles per second).
Even inching toward the speed of light is difficult - immense energy is required to get to even a fraction of the speed of light, and the closer you get to the speed of light, the more energy is required. The amount of energy you'd need to achieve the speed of light is infinite (i.e., more than you've got, even with those supercool long-lasting batteries). So just tossing in a few more dilithium crystals into the warp drives isn't going to make it happen.
There are loopholes in our understanding of the physics that make faster-than-light travel theoretically possible. For example, it's theoretically possible to create a "bubble" of space that breaks itself off from other space and moves faster than light relative to that space (all the while everything inside both "spaces" moves no faster than the speed of light). This is known as an Alcubierre Warp Bubble. The catch (there had to be one) is that these bubbles require the existence of exotic matter that has negative energy, and wouldn't you know, there isn't really any lying around, and it's not clear that any actually exists.
3. Laser Bolts You Can Dodge
Aside from the issue of Imperial Stormtroopers being bad shots, let's review a fundamental fact of light (which is what lasers are): It travels at 186,000 miles per second. So the idea of ducking before the laser hits you is just plain silly.
Not to mention (of course) the idea of a laser bolt being visible as a streak that has a beginning, a middle, and an end. If you were zapped by a laser from a laser gun, it would look like a single stream of light, with one end attached to the barrel of said gun, and the end attached to whatever portion of your head had not melted yet (assuming you're having a laser battle somewhere where there is enough air around to illuminate the entire beam).
Most "laser" beams in science fiction movies travel slower than bullets do today. Let's see Obi Wan whip his light saber around fast enough to stop the spray of a Mac-10 (and let's not even begin to talk about all the things wrong with a sword made of light).
4. Human-Looking Aliens
This is endemic on the various Star Trek series, where creatures from entirely different sectors of the universe look just like humans except for the occasional bulging ridge on their foreheads. Yes, this is the result of having only humans at casting calls, but in a large sense, all these "humanoid" variations ain't gonna happen.
Look, humans evolved on earth and shared a basic body format (four limbs, one head, side-to-side symmetry) with just about every other vertebrate on the planet. It's a form that works fine for this planet, but not even every vertebrate sticks with it (see: snakes, whales, seals, etc).
Given that any planet with life on it will have that life evolve in it's own way, the chances of the universe being stocked with chesty alien princesses who crave human starship captains is slim at best.
Related to this is the following.
5. Half-Breed Aliens
Humans don't even interbreed with other species here on earth. Our DNA is simply too different from other species to allow such a mating to produce offspring.
Given this, what are the chances of successful mating with an alien species that may not even have DNA as its genetic encoding medium?
Also going back to the idea that aliens probably won't look like Humans, how would you do it anyway? It's not exactly the "Insert Tab A Into Slot B" proposition it would be here at home.
6. Brain-Sucking Aliens
Ditto aliens that control your body by using your brains, or gestate in your chest, or whatnot. Let's posit that any creature that controls the brain of any other creature (not that any exist here on Earth) does so only after a few million years of what's called "speciation" – i.e., one species eventually enters a symbiotic relationship with another species. This relationship would have to be pretty specific, as symbiotic relationships are here on Earth.
Which is to say just because you're in a symbiotic relationship with one species doesn't mean it transfers over to another species, especially an alien species, who's body chemistry, DNA, brain wiring, etc., isn't even remotely close to your own. So don't worry about the "Puppet Master" scenario too much, or that you'll be nothing more than a glorified egg sac for some nasty breed of space monster.
7. Shape-Shifting Aliens
Shape-changing aliens are all very well, but there's a tiny problem in having a roughly human sized lump of alien protoplasm turning itself into, say, a rat, to scurry around in the ventilation shaft: Where does rest of the alien go? You can't just make 99% of your mass disappear into thin air (or reappear, as the case may be); it has to go somewhere.
Unless that "rat" is running around with a highly compressed mass of a human-sized object (which presents its own problems), shape-shifting in to different sized objects is not very likely (one of the smart things about Terminator 2 was that the T-1000 only shape shifted into things of roughly the same mass, like human beings or a floor).
8. Time Travel
Got an itch to spend time in the Arthurian England? Or perhaps Gettysburg during the Civil War?
The same relativistic principles that keep us from going faster than light also keep us rom traveling backward in time and messing with the past. It's possible to slow down time - the closer you get to the speed of light, the slower time moves for you relative to your original frame of reference - but to get the clock spinning in the other direction would require you to go faster than light, and you can't do that.
Again, there are theoretical loopholes that could allow it - worm holes, actually, which are "tunnels" in the fabric of space-time that could possibly allow travel back in time. but once again, keeping these wormholes open would require exotic matter with negative energy. Got any? Neither do we.
9. The Planetary Gravity Scam
Everywhere you go in science fiction, people are walking around like they weigh just what they do on Earth. Chances of that happening in the real universe? Slim. Consider our own solar system. On Mars, a 180-pound man would weigh just 70 pounds; on Jupiter, 424 pounds (not that you can walk on Jupiter, as it has no solid surface). That man on the moon? Just 30 pounds. The man's mass is the same, it's just that different planets have different gravitational pulls.
The idea that all the planets that humans might visit would exactly match Earth's own gravitational profile is a little much. As is, alternately, the idea that all alien creatures would be as comfortable in our gravitational field as we are.
10 The Planetary Sameness Principle
Tatooine looks just like the Yuma Desert in Arizona. Actually, it is the Yuma Desert of Arizona! Photo via Wookieepedia
The desert planet of Tatooine. The ice planet of Hoth. The jungle planet of Dagobah. What do these planets all have in common? One planetary-wide ecosystem. Which isn't too likely.
Our own planet has varying zones and ecological areas: desert, tundra, jungle, and so on; other planets in the system also show marked zones of varying atmospheric and weather patterns. Mars has ice caps as well as (relatively) temperate zones; Jupiter has distinct weather systems based in different areas on its globe. The planets that show a sameness are the ones we couldn't live on. Venus is all desert, but that's because a runaway greenhouse effect makes it hot enough to melt lead. Pluto is all ice, but it's so far away from the Sun that its atmosphere freezes for most of its orbit.
There may well be purely desert or jungle planets, but most planets we'd want to live on would probably be able to accommodate both.