District 9

The critics loved this one. I saw it called the "thinking man's science fiction movie", and everyone was impressed with the idea of aliens that were basically just people, even if they looked like soot-covered giant shrimp.

But there was a plot hole in this movie, a hole big enough to steer the Mothership through, and nobody but me seems to have noticed it. Since it's a plot-driver, it should have been fixed.

A big deal is made of how people can't use the superior alien weaponry since the weapons only work for someone with "prawn" DNA.

Okay, it's likely that within this century, humans too will have weapons that test the DNA of the holder and, for obvious security purposes, won't fire if the wrong person pulls the trigger. But how likely is it that we'll make such weapons with the default setting "work for anyone but only if they're human. No space aliens allowed, please."

How likely is it that we'd even think of such a preposterous setting? Or that the aliens would?

If the alien weapons (and mothership steering console) test DNA, it's so they won't be used by the wrong aliens. The idea of setting them up to check species (and *only* that) is just plain silly.