Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Sci-fi movies come in two major flavors: 1) they happen today, in our world, but have all sorts of aliens to make things more interesting 2) they happen sometime in the future, usually far away from Planet Earth and introduce all sorts of cultural and technological changes, as well as a plethora of alien races.
Still, regardless of their genre, they all have one thing in common - the same classic mistakes over and over again.
Mistake 1: Aliens speak English
I know the language is there to make the viewers actually be able to understand the plot, but it's much more than that. The problem is not with the words - Google can do that. The problem is with the understanding. Unless you're human and have lived on Earth for the last few thousand years, being able to understand the nuances of what is being said, regardless of the actual words, is nigh impossible.
You don't have to be an alien to find difficulties in languages. How do you explain cases in Slavic languages to someone who's never used them? How do you explain the 56 tenses in French to a Chinese? What about the simple phrases like Bob's your uncle, Plonker and cushty? Even Americans have problems with those and they speak English, so imagine the cultural shock of an alien landing on a Scouser.
Mistake 2: Weapons
For some reason, all weapons in the future or those used by aliens are particle weapons. Not bad, except the simple fact that focused energy beams (e.g. lasers) dissipate in air rather quickly, making the weapons rather useless.