1. Global Warming Can't Flood The Earth
While it's undoubtedly true that all of the world's ice caps and glaciers melting at once would have a disastrous impact on the earth, the idea that it would literally flood every piece of land is, fortunately, quite ridiculous. If the unthinkable happened, sea levels would rise considerably - by about 200 feet - devastating coastal communities and causing irreparable damage, but large cities inland would remain unharmed. This false idea of entire states being turned into seaside resorts overnight is perpetuated by more-than-slightly ridiculous blockbuster movies; but even more concerning than that? Plenty of people believe it's possible.
2. Laser Beams Are Invisible In Space
Sorry sci-fi fans, the truth is you can't see lasers in space. In fact, you usually can't see laser beams at all, wherever you are. Most lasers that we come into contact with are inexpensive and weak, and they don't emit enough photons to be visibly interacting with particles in the air - dust or some other kind of residue. Those tiny, pen-shaped lasers, for example, that produce a dot on the wall but not much else. Take a laser to space and even the most powerful beams will remain invisible because dust particles in that near-vacuum are so spread out. This is a fact that most movie directors choose to ignore, however. Why? Because invisible lasers are nowhere near as fun, that's why!
3. Removing Your Helmet On Another Planet Would Be Foolish
Many movies make the assumption that once their protagonists discover another planet that has high levels of oxygen, they are safe to take off their sweaty space helmets, get comfortable and kick back with a bag of chips. Not so fast. Even atmospheres rich in oxygen can be lethal, for a number of reasons. First of all, too much oxygen can kill you. Second, there could easily be other harmful gases present that would lead to a similar fate. Finally, breathing in microbes from an alien ecosystem would likely cause severe damage to your body. Basically, taking off your helmet on another planet just isn't an option. Sorry, Hollywood.
4. You Can't Hear Noises In Space
Unless film directors are willing to go down the silent movie route, most space-themed scenes will have to be considered unrealistic. Why? Because sound is a form of energy that requires air to propagate. With space being the vacuum that it is, sound just isn't present. Great for those who enjoy peace and quiet, not so great for fans of intergalactic battle scenes.