So when a creature flick comes along that is so preposterously ridiculous, so patently absurd, so monstrously unscientific ... well, we don't actually notice until years later when we need a premise for an article. But then we have no choice but to feel betrayed and scream "BULLSHIT!" at the top of our lungs.
As seen in: Alien
The Facehugger was just one of several horrific stages in the life cycle of the alien race in Alien. Basically, the thing was an octopuss-looking abortion of squirming slime, whose whole purpose was to implant the next phase of itself into a host. And it did so by facerape.
It latched on to a guy's mouth and pumped its offspring down his throat, which later burst out of the poor fool's chest.
Why We Call Bullshit:
What do aliens eat? Do the movies ever establish that? Well, they'd better eat the exact same damned diet as humans, since their offspring apparently have to survive on the exact same nutrients found in a human body. After all, a human embryo/fetus requires an incredibly specific set of nutrients, so much so that artificial wombs are still science fiction. And keep in mind, this species didn't evolve specifically to use humans as hosts--as far as we know, they had never encountered humans before the events of the first film.
Alien tries to bullshit its way around this by saying that the Alien's DNA merges with the host; this becomes totally ridiculous when you realize that, despite sharing a good 99.9 percent of our DNA with chimps, we're still genetically incompatible with the fuckers. Since the Alien is way, way less like us than the chimp, this would be like saying a Komodo Dragon could successfully impregnate a human.
But even that analogy breaks down when you realize that the Alien was not only completely different from humans, it was completely different from every living creature on the planet. Yes, unlike carbon-based life (a.k.a. fucking everything), the Alien was a silicon-based creature: At one point in the movie, we see it replacing its exoskeleton with polarized silicon, which would be like if you replaced your skin with glass.
To update our comparison, the Facehugger's reproductive method would be like if your window came to life and made passionate love to you, and then a week later you gave birth to shot glasses.
As seen in: Return of the Jedi
The Sarlacc, for the zero of you who've never seen Return of the Jedi, was a desert creature that strongly resembled a sandy, tentacled anus. It lived mostly underground, except for its gaping pothole of a mouth which, along with its 37,000 layers of teeth were exposed in the hopes of securing some clumsy grub.
Why We Call Bullshit:
There's no way the Sarlacc could get enough sustenance to survive.
First off, the creature you saw in Return of the Jedi--presumably an adult--was completely immobile and relied on prey just rolling down into their mouths, sort of like the pitcher plant. Except the pitcher plant (A) can blend into its surroundings since it looks like another plant and not like a massive hole in the ground, (B) it doesn't live in a fucking desert, so there are actually plentiful insects for it to trap and (C) is a plant, so it has sunlight and photosynthesis going for it. The Sarlacc has none of that.
Enjoy your sandwich, Sarlacc.
Also, the pitcher plant isn't fucking enormous. You can't run the metabolism on a creature that size without a massive amount of food. "But wait," you say, "it's never really clear how large the Sarlacc is - it was buried in the sand, for God's sake!" This is Star Wars we're talking about here. Professor Wookiee Hans Deetoo has it covered:
Can't you leave anything to the imagination, nerds?!
See that tiny bit up there with the tentacles? That's the part you saw in the movie. Turns out the vast majority of the Sarlacc is underneath that, sort of like an iceberg--except icebergs don't need to eat.
According to that image, the mouth is three meters wide; assuming it's to scale, the damn thing was roughly 45 meters long, making it way bigger than even the blue whale, an animal that needs up to four tons of food daily.
Wookiepedia tries to bullshit their way around that by saying that the Sarlacc digested its food over a thousand years, which would be like saying that you could survive on a single Oreo for your entire adult life as long as you ate it slowly enough.